Why do we even do mens vs womens sizes for clothes?

You must be man. 😀
Me too, but I also sew, so: Different types of clothing fit women differently based on size. And I don't mean body shape fit just being slightly different. A "small shirt" can have a million different lengths which could reveal midriff based on bust or Shoulder size. This is impacted by curves but is a total dofferent measurement. And this can happen with any gender, but statistically more so for women.
And wait until you realize that people are sized different in different countries other than America.
The best solution would be to remove sizes altogether and create a standardized measurements sheet that is required to be included on clothing sites or tags. Or at least an international standard. It's done in engineering, why not in textiles?

Shapes need to be different, as others have already said. So men's and women's clothes do need different construction. But sizes could be just measurements. Absolutely. Yes.
Shoes, no. They should all be length in cm and then a narrow, medium, wide, xwide.

Because men and women have different body shapes. A small man and woman may need the same size but certainly not the same cut.
E: nvm i skipped over you talking about body shapes. Yes I'd agree that there should be more options, but even an overweight man will likely need and want a different cut than a woman of similar proportions. So the distinction between genders still makes sense, just that each needs to accommodate more body types.
- Nowadays all women's clothes are straight up and down, the only difference is pink. It's a nightmare for women who are shaped like women.
  
  The fuck is a "woman-shaped woman"?
  
  Ehh I've had to use women cuts shirt ss undershirts sometimes, even if the size was right they always felt tight across the back and shoulders
  
  That's just not... true... Maybe the ones you choose to buy 🤣
  
  Are women who fit into the clothes you describe not women?
  
  Yup. I agree. Men's pants more often accomodate my wide ass better than women's clothing. I don't know how. It shouldn't be possible. Even the swimming trunks fit better... I found out ball hammocks are a thing. Not out of curiosity, but out of desperation! And sometimes, I find pants that fit my ass, but the legs are too short!
  Another thing is that women's clothing is meant to be form shaping. So if you are less curvy, the clothes got you covered. If you are too curvy, the clothes will crawl and migrate down the hills.
  It's hard to find practical clothing. Such as a swimming top that will stay the fuck on if you jump off a cliff. Or a bottim that has space for more than 1 asscheek. Let alone to avoid the camel toe.
  Not to mention flimsy fabric with shitty patterns added. Found a comfy nice shoe? Well here is 2cm^2 of snakeskin pattern for no reason. How about a nice shirt? Got some stupid white woman quote on it though. Or an animal with massive eyes to make your boobs look bigger... ffs. Or the bad design? Nice hoodie? Well it's short enough to show the belly button. Pockets? Some are fake. This nice looking top has 3 straps and you need to fold it inwards to make it stay in shape! Oh found a nice, 100% thick wool jacket? Oh it's men sized. Arms are too long and a bit to tight around the bottom. Well let's see- oh the women's section doesn't even have the option. Only a thin fleece hoodie in bright pastell colours.
  God I swear to you, I hate women's clothing so fucking much.
  "Maybe I should just go swimming at a nude beach... that way I won't have to search or worry anymore."

Generally, women are going to have boobs and wider hips and men generally have bigger feet. Obviously there's a massive spectrum of human body shapes, but this is the main differentiation.
If I wear my (male) partner's t-shirts, they're just straight with no space for boobs and they don't come in at the waist. Same with trousers, there's a massive pouch in the crotch, too narrow on my hips and too loose at the waist. People like to wear things that best fit their bodies.
- Stuff like ski boots, too. Men and women apparently have muscle groups placed in slightly different areas, so the boots are sculpted differently to account for that.
- Style/cut is different than the measurements though.
  
  It isn't about style. Women's sizes (e.g. 8, 10, 12) originally came from codes based on the variations between hip, waist and bust measurements.
  
  It's a shortcut for measurements.
  It's easier to shop "Men's L" or 32x32 pants than remember a lot of measurements.
  I think it's different if you want your clothes to fit a specific way, but for my shopping I want to minimize the time spent shopping and get "good enough". That's how we end up in minimal measurements.
  I think ideally we'd have both sets: the minimal measurement and the maximalist ones for the same garment.

Tradition plays a huge part in it.
Why do so many women's dresses zip up the back? Because servants used to be a thing. Same reason women's shirts button differently than men's; someone else was supposed to be dressing the lady.
Even fifty years ago in New York a man could be arrested for wearing something the police considered "women's clothes." and NYC was considered a loose town with easy morals
- The zipper in th3 back also helps if you have a big chest and don't want it bifurcated by a zipper.
  And you can't put that same zipper (if it goes all the way up the back) on the side... if you have wide hips.
  So at least that still has reason to exist.
  Source: my own body and never wearing a front-faced zipper after seeing why we don't do that.

Well, depending on who you ask, vanity or capitalism.
Women's sizes were chosen.
Men's sizes are typically measurements, usually waist × inseam and neck × sleeve × chest
Women's sizes are harder because they have a wider range of measurements needed to get a good fit. Bust size, waist size, and hip size can have different ratios to each other, even when the waist size is the same. So, Wanda might have a 20 inch waist, a 58 bust and a 36 hip measurement. But her sister June might be 20 waist, 36 bust and 58 hips. Obviously, clothing measured the way men's is wouldn't give as reliable a guide.
So, way back when mass produced clothing came onto the scene with standardized sizes, something needed to be picked.
Turns out that a size 6 sells better than a size 20, even if the actual measurements are exactly the same. Not that men's clothing is immune from vanity capitalism, you should see the clusterduck that is XL sizing.
But, with "dressy" clothing, mens shirts are usually going to be measured. Women's sizing, particularly dresses and pants, they go by the fairly arbitrary numeric system based on ratios. Just don't ask me how it was calculated originally, I never cared enough to find out.
Thing is, while those sizes were originally meant to standardize things, that no longer works. You go get a size 6 in one store, hold it up against a size 8 from another, and they'll be the same measurements. Why? Because they're playing a numbers game based on vanity. Some places, a size 6 is unrecognizable as an actual size, it's just so far off from the median.
Also, I use size 6 a lot because it was, at one point, the "default" size for models and mannequins. I think that's changed, but it stuck in my head, so I tend to pull it up as a baseline example. I know it's usually what's used for fitting models, which is a whole thing of its own. It varies a lot more nowadays for runway and catalog work though. Height is more important in catalog work afaik.
Anyway, tangent aside, shoes are a bit more practical. Women's feet have a slightly different set of angles, so just a toe-to-heel measure wouldn't work exactly the same between men's and women's feet. I can't recall the exact points where measurements occur to get the different sizes, but that's what it comes down to. You have to measure the feet differently to get a good fit.
Which is the overall why none of the clothing sizes will cross over well.
Yeah, a men's XL is going to fit a woman with a given bust measurement about the same as a women's xxl (iirc, don't hold the exact conversion as fact, I'm just pulling from memory here), but they may not fit the same.
A men's dress shirt is going to fit a woman horribly, even if it's the right chest or neck diameter. It'll be cut for a bigger waist, with longer arms. But a woman's dress shirt will fit z better*, because that's taken into account.
Funnily enough, men that lift a lot of weights end up having trouble fitting men's clothing sizes as well. You get something that fits your chest, it won't fit your waist (unless you're a power lifter, where you tend to see less difference between chest and waist than in bodybuilder circles), and it may not fit your neck worth a damn. Buy for the neck size, your sleeves can be baggy.
The patterns used don't scale up the same as the human body does as it puts on muscle. It's still not as big of a pain in the ass as it is for women with significant differences, but it is a pain in the ass lol. I've never been able to buy a suit off the rack. I've only had a few, but they all had to be tailored.
- Funnily enough, men that lift a lot of weights end up having trouble fitting men’s clothing sizes as well. You get something that fits your chest, it won’t fit your waist (unless you’re a power lifter, where you tend to see less difference between chest and waist than in bodybuilder circles), and it may not fit your neck worth a damn. Buy for the neck size, your sleeves can be baggy.
  It is even worse that that. There is plenty of variation in body types and how manufacturers design their clothes so that being fit or just slightly fat has less impact on whether something fits than whether they even design for your body type.
  I tend to avoid long sleeves because I have lengthy gorilla arms, which hasn't changed even as I've put on weight over the decades. Hard enough to find something that is long enough for my torso.
  
  Yeah, that's part of why I shifted to baggy stuff. More comfortable, easier to shop for.
- Yeah I have shoes ranging from 46 all the way to 51 on the french scale for size. (They all fit me about the same). Same with tshirts ranging from M to XL pretry much the same size.

Take shoes for example. Why is a uk men's size 10 so wildly different from a UK women's size 10?
EU shoe sizes aren't different based off gender; women just tend to have size 37 when men tend to have size 42.

Desired profile and fit are different.
I “passed” before HRT often just by wearing men’s clothes. The way that pants shape your butt is very different. I occasionally thrift women’s blouses and think I pull them off well, but these are ones from the 80’s and 90’s with the more “triangular” frame.
Women’s sizing is an incoherent mess. Men’s sizing at least in theory should be based on measurements. Historically, you would have been expected to have your clothes adjusted by a tailor anyway - this world of fast fashion, “ready to wear” means most of us are walking around with clothes that fit us poorly. Mass produced clothes are trying to fit some sort of “average” person, and none of us have a perfectly average body.
As far as shoes - there’s differences in shape and men’s feet on the whole tend to be larger. I think toebox proportions are different.
(The real danger of a “mixed gender” shopping section would be people realizing how shit women’s clothing is. Cheap flimsy material, lacking pockets, constructed to fall apart with a stiff breeze - designed to be disposed of. Shoes being designed with no thought for comfort or long term health…)

Have you seen a woman? Their bodies are different, clothes are cut differently. What the hell man. Im surprised this is a serious post. :)
How are you going to go into a shop and get clothes for your "body shape" in the scenario that there are no male or female clothes?
You can do this if you custom order a suite from a tailor but not when you shop for sweaters in the shop....
There are a million things that only women wear also.

This really depends on the clothes. Shoes should absolutely have a single sizing system and I would love for all pants to use waist/leg measurements like jeans do. But many other garments are completely reasonable to separate based on gender (or sex, rather), primarily tops due to the fact of boobs existing and even pants like I mentioned are able to have tighter crotches on a women's size.
- Shoes are pretty easy to guesstimate for US sizes if you have average feet (width being the biggest reason this fails). Men size is basically women size plus 1.5. This doesn't work if you have a large foot size though, since women's dont go up to the same max as men's
  Clothes are 100% a pain. Especially online shopping
  
  I just wish there wasn't so much guesstimating required. I stopped growing a while back and yet I've fit an 11, 11.5, and 11W, depending on the shoe. I wish it was a bit more standardised based on length and width. I've never shopped online for shoes because I know they probably won't fit right!
- In many parts of the world, shoe sizes are unified (cm or mm). They still use the labels for men and women for style but, so far as I know, they're otherwise the same (unless the widths differ, but those are also standardized in many places so you might get like a 25.5e for 25.5cm width e)
- My man-boobs would like a word.
  
  Usually you want to hide your man boobs, women's clothing usually aims for the opposite.

I mean, a men's size 10 is wildly different to some other brands size 10

Like others already mentioned, they don’t just differ in size, but in shape as well. What I would prefer is different varieties for each size per sex. For example, I am an Asian medium. So when buying shirts in the US, I’m in that weird spot where a US medium is too wide on the shoulders and the sleeves too big, but the torso is just right. On the other hand, a US small fits just right on the shoulder width and sleeves, but the torso is too tight. It would be great if there are variations based on build as well like pants (i.e. slim, standard, wide, etc.).
- That's why I like Levis pants. Lots of different fit options
- Sounds like I am the opposite of you, as I have broad shoulders and long ass gorilla arms and torso so I end up needing to hunt down the long/tall version of XL sized clothes for most brands.
  I would love something for regular clothes that was more like dress clothes with multiple measurements to work with like sleeve length, shoulder, neck, etc. to more easily find what brands design for my body type. Hurley for example is an excellent fit the vast majority of the time for me, and I'm not a cut sports guy.

Tell me about it. It’s an exercise in frustration to find good fitting clothes as a trans woman because most clothes don’t come with a sizing table and if they do they skip the most important measurement for me: shoulders.
Or it can be a real punch in the gut when the description includes what size the model in the photos is wearing. For example the model is the same size as me (1.74m) and wears a size S. And I’m like great I will take that in L or XL…

I would have to speculate, but I think it's due to averages.
The average woman is much smaller than the average man. There are a handful of sizes that will fit most women, and a handful for men. Think of a standard distribution: there will be outliers, but most are in a certain range.
Over many decades, even centuries, sizing is more rigidly codified as larger scale production of textiles/shoes becomes common, but of course this sizing doesn't apply to everyone for various reasons, so things such as vanity sizing and regional/manufacturing differences created an even bigger mess.
Not to mention that people are fatter now because of the caloric density of modern food, so it's relatively common now that even really short people can be very wide. Plus sizing is also a huge clusterfuck.
Another factor is fit and cut, but this comment is already too long to get into that.
- I worked for a clothing store in college. I leaned that each store / clothing producer has an ideal body shape they cut the clothes for, with some variation. The higher end store you shop at, the fewer variations.
  Higher end shops usually cater to a specific body shape and size. Those clothes will fit a handful of people extremely well. Big box stores will have clothes that fit everyone, but that fit will be so-so for everyone.
  
  Yeah, it's kind of annoying that women basically have their clothes sizes randomized and you have to look at their size chart online anyway.
  At least when I go buy pants (as a man), I can reasonably assume that almost any pair I get off the rack is labelled by hip circumference and instep and roughly the size I'm expecting. Though there's a decent amount of vanity sizing in a lot of men's clothes these days too.

Resisting the urge to downvoted because this is a great question for this sub.
A lot of it is historical and never changed for 100+ years. Same reason buttons are on opposite sides on men and women garments.
- Why do you have the urge to down vote?

Marketing

Many things like shoes could easily be for both men and women, like for instance sneakers.
But with the different size standards, that can't work.
The result being identical sneakers have a series for women and another for men! Although usually the colors are different. But what if we would rather have a choice of ALL the colors?
It's not just stupid it's moronic.
- Even if the length is the same, men's shoes tend to be wider than women's (sometimes much wider). So swapping is great if you're a woman with wide feet, but if you're a man with wide or even average feet, it would be much harder to swap to a woman's shoe.
  
  womens comes in a narrow and wide versions. if you need wide, wouldnt the wide versions of either work? im right down the middle and never had an issue with width in mens or womens.
  
  OK I didn't know that.
- at least in the us, womens [generally] are just (men - 1). theres no reason we could not just have a single set of numbers for shoes.
  
  Minus 2 in US

To charge women more? The pink tax is real

Because gender was invented to increase shareholder value
_/joke

It depends on how the standard was made, with the decisions generally made to fit specific reasons and no overarching standard.
There are also clothes that use more than one number. A lot of mens pants use two numbers; the same with formal mens shirts.

No gꝏd reason for it , don't listen to the biœssentialists in this thread that say otherwise
Degender (body types|clothing) NOW ‼
EDIT : wording
- Bravest comment
- Most ridiculous thing ive ever heard.