Why do car repair shops always assume you can leave while they work on your car?

But how can they _assumef that I can even afford a cab, well Uber these days, when I’m about to have them hundreds of dollars getting my busted-ass, POS car fixed?
Because they're not your parents, and its not their job to be responsible for you between the time you give them your car and when they contact you to tell you its ready for you.
The shop's statement of "you can come back in 3 hours to pick up your car" isn't saying "we know you have other resources for transportation during the time when we're working on your car" its the shop using a social shortcut to say "your car is unavailable to you for 3 hours while we work on it, and we have no need of you until then. You can go away for 3 hours and it won't interfere with our work".

Because that's what 90% of their clients do.
Why does a realtor/bank make you put your address on the application to buy a house? If you're buying a home, why would they assume you already have one?
Honestly, I don't think they "assume" that you'll do anything other than give them a car to work on and pick it up and pay when they are done. Whatever happens before, after and in between, isn't their problem.
- Adding on to this: the repair shop I take my car to is too far from my house for me to walk or bike back, so I just walk the shops in town while they work on my car (unless they tell me ahead of time it might take more than one day to diagnose+repair, in which case I ask a friend to drive me back home after dropping off the car).
  It's less that they "assume you can leave", but rather that it isn't really their problem. They need an uncertain amount of time to work on your car, depending on the issue being repaired, and you can leave if you want to during that window.
  If there's nowhere for you to walk/bike to nearby, you just gotta sit and wait, which I've done on a handful of occasions. Just sitting in the lobby and reading some outdated magazines for an hour or two. It's boring, but what can you do?
  
  Adding on to this: the repair shop I take my car to is too far from my house for me to walk or bike back, so I just walk the shops in town while they work on my car
  in washington, oregon, california, nevada, arizona, new mexico, texas, illinois, new york, new jersey, pennsylvania, georgia and florida; the best car shops tend to be in the industrialized areas with no shops nearby and while the most overpriced car shops are nearby other shops. i hope you're not spending too much $$$ on your repair bills.
  
  In addition to this, I also added into my post that there is no guarantee they will even look at your car today or tomorrow. Other customers and problem difficulty can delay their timeframe to even start diagnosing.
- Whatever happens before, after and in between, isn't their problem.
  OP is looking for Jim Bob's Auto Repair & Adult Daycare
  Clearly an overlooked business model

Do you get shitty at pilots for landing the plane and just assuming everyone on board is good to get home from the airport, or would you like them to pay for 200 cab fares as well?
It's their job to fix people's shitty cars, not their shitty life admin skills.
- Hell yeah, I'd like them to pay me £200 to get me home. Who doesn't want free money? ✈️
  
  Then the tickets would go up by £200 each, and you wouldn't have the option to take the train or something else.

They don't. My local shop has a waiting room with coffee, sodas, Wi-Fi, and seating/desks. Plenty of people (myself included) bring their work with them to the shop while their car is fixed. Also I've definitely seen people driving around loaner vehicles from dealerships before which is kinda a solution?
But you're right that it sucks that our society has designed places where it's impossible to get around without a car. My car shop is a short bus ride from my house or a slightly longer walk. If I take my car to the dealership, which is farther away, home is a bike ride away or I can go to the shops nearby. I'd guess this is the case in most denser urban areas except maybe some US cities which are just terribly laid out.
- A independent car shop I know has created a small "Coworking space" along the usual waiting area. If you bring your car for a half/full day repair you can book one of these spaces for a small fee (5 or 10 bucks). (he even offers them for a small price for external customers if he has capacity).
  It includes small offices (full wall,not cubicles), Wifi with a fiber uplink, etc. and works really well for him - a few major companies around here switched towards his shop for their fleet.
  We sadly can't use him, as we have a long term rental/lease and that requires a network shop of the brand.
- I'm new to car ownership and I wrestled my bike into the back of my low-roofed saloon car to cycle back. I didn't really buy the car with cycling in mind but it beat paying them £25 for a courtesy car (I expected not to have to pay for that is this was to fix a recall issue)

The same car mechanics that usually have seating, coffee, TV etc for people who can't leave to use while they wait..? You can also idk walk somewhere and grab a bit to eat? Don't be mad at the mechanic because your car is broken and you don't know how to fix it lol, also most shops that I've been to have offered me a ride when I live in the area and they aren't too busy

They assume that, by driving your car to them and paying them to repair it, you are an adult who can actually figure shit out for yourself.

You CAN leave. If you have one car and that’s your mode of transportation then you realistically have only a few options to supplement your handicap of being car less:
ride share
gig economy (doordash/grubhub/etc and Instacart/gopuff, etc)
work from home
public transportation
rental car
loaner car if at a dealer and they have any available
take sick days until your vehicle is repaired
What would you do? The repairs depend on what is wrong and sometimes those symptoms can be a menagerie of things that require troubleshooting what the actual root cause is. That takes time. Not to mention they may not even be able to check your car today at all. They have other customers with similar or worse issues all wanting their vehicle fixed ASAP as well and might also be a one car household.
Do you have insurance? See if they will cover a rental for you while your car is in the shop. Otherwise, find one of those other options above and figure things out.
Yes, a car is expensive and when it goes down then you are forced to find other ways to ensure you can make it.
- I've also thrown my bike in the trunk and then biked home. Easy option for those able to bike and live close enough. Also, lots of cities now have bike share stations all around.
- Also, you know, getting friends and family to give you a ride.

What's the alternative if it's going to take hours/days to fix your car? It's not so much of an assumption as it is a necessity. Some dealerships have loaner cars but that also comes with wildly inflated repair costs.
- I can only assume OP wants the shop to taxi them home. maybe back when the car is ready? Most fixes on the range of hundreds of dollars take several hours, or days depending on the availability of parts/tools.
  Shops fix cars, driving customers is an added value I wouldn't expect from most car shops. More so, if you committed to pay a few hundreds for a repair when it's ready, it's reasonable to assume you have enough money to go home on your own. A bicycle is always an option, though.
- My repair shop has several loaner cars. But loaner is a real strong word there. They’re about the junkiest piles of rust you can drive lol. But they get you home and back to the shop! And they’re pretty dang cheap too.

You can still walk, can't you? (PleaseHaveLegs,PleaseHaveLegs,PleaseHaveLegs...)
- Lemmy users are mostly Americans. You can't really walk anywhere in the US unless you're in a mall or a long distance runner. Everything is spread out.
  
  With 4 lane roads and 80 km/h traffic separating anywhere you would want to actually walk.
- Place I take my car too, 10 minute drive... 3 and 1/2 hour walk one way back home. Or I could go to the nearest town which is 2 hours and 50 minutes away the other way.

That’s the advantage of taking your car to the dealership, the dealership in my area gives you one of their brand new cars from the lot to drive around while you wait for your car to be fixed. It’s a sly trick that eventually works and gets you into a new car.
- This is how I found out that Subaru's adaptive cruise control from their 2020 (iirc) models is vastly superior to the adaptive cruise control on my 2017. It practically drives the car for you now.
  
  Yeah. Because that's just what the roads need. Drivers paying LESS attention...
- The dealership I go to did this last time I needed a lot of work done and all it did was make me dread the day my car finally dies and I have to get a new one. Spent the whole way home fighting the stupid lane departure thing that was trying to force me into potholes and road debris. I was almost home before I figured out how to turn it off.

A big reason for it is you bring your car for service. It's going to take time. They have a bunch of cars to get through, they'll look at yours, determine what you need, sell it to you, then order parts. In the meantime the tech has moved on to another car while they wait. You're sitting in the waiting room wanting to know WHY IN THE HELL IS HE NOT WORKING ON MY CAR??!?!?! I'M SITTING HERE AND YOU'RE WASTING MY TIME!!! Then you're mad it took half a day to get the parts in because the parts supplier had to run across town to get the parts and you leave a bad review. That bad review wasn't fair and it hurts the shop.
At the end of the day, it's not worth working on your car under those circumstances. You admit your car is a POS and you're broke. That also means your car is going to need a lot of shit you can't afford so you're going to pick the bandaid to keep it going. Then a few weeks later something else breaks because you put the bandaid on it and now you're mad and blame the shop. More crap the shop has to deal with when it wasn't their fault or problem in the first place.
- That's a lot of assumptions to not even answer OPs question.
  
  How long have you run a shop?
- Ah so you've had your computer repaired before.

Perfect timing on this post!
I chose my “normal” garage partly because I can walk to it
I occasionally have friends or family who can help
otherwise I’ll just wait
However for the first time ever, I just got a loaner car from the dealer. Their estimate was they’d be done by last Wednesday but they haven’t started yet. Meanwhile the loaner is equivalent to my vehicle and it’s warranty work - they’re just making it damn expensive for themselves
- The dealership probably gets reimbursed by the manufacturer for that loaner (and the warranty work).

Where I'm from most shops loan you a temporary vehicle while they work on your car. You only need to top the tank when you tske it back. Very convenient. Not that I could even afford to own a car myself.

What would you like them to do instead? How much extra are you able and willing to pay them so that they can do that and stay in business?

I live in a Finnish midsized town with a population of roughly 80 000 people. My repair shop offers to drive you where you want to go, but you're in charge of the way back to the shop when they call to say your car is fixed.
It's a great service, and it guarantees my loyalty to that shop!
- We have some loaner cars and will drop you off and pick you up if needed. It really is a cool thing and the word of mouth alone has gotten us enough business it's almost nuts.

Some places DO offer to take you somewhere, which is super cool. But face it, to take you back home and then bring you back again when the car is finished is cost prohibitive for a normal shop. They would have to have at least one extra vehicle to do this, a dedicated driver and insurance to cover what is essentially a completely separate service. They are a car repair service, not a shuttle service.
- They use a little shuttle van. Hell, this was a thing in New Jersey, so I'm surprised when it's not also in every better state.
  
  But not at every place right? Hell, I have one car right now that still has a warranty and they give me a rental car. It's awesome service but you don't get that from a smaller place.

What this means is you actually can’t afford to own a car and are trapped in the corpse of a long dead society that once would have enabled you to own a car.

Yeah I feel you here. They kind of look at me like what you're going to stay? As if I'm the weird one for not having a personal driver to come get me at all times of the day. Like bitch no I don't have that option. My options are sitting in your lobby or Trek miles through the 110° heat back to my home.

Most people call in someone who can take them somewhere else and bring them back when the car's ready

Most people have a limit to the amount of time they're willing to sit in a shop lobby.

I'm leasing a car. I went for service last week and they had free coffee and coffee bread while I waited.
I think that should be put in law for all repair shops. :) It's the little things.

My shop usually offers to drive me somewhere. Also helps to live in a country with viable public transportation. And higher tier shops even have cheap car rentals which often even tie into people's insurance.

I usually try to plan a few foot-based errands in the area while they work on my car: maybe the pharmacy for my meds or some toothpaste, the bakery for a couple of these cupcakes my daughter loves, browse the bookstore, talk with the tea shop owner.
We're always in such a hurry and complain about missing the 'community feeling' of 'the old days', yet we never spend the time to just walk about the community, doing errands instead of "running errands", casually catching up on events and goings-on. I like to use my time for that kind of thing.
- I think this might read as a little tone deaf to the economic reality of many people.
  Also mechanics are often in semi-industrial areas where the most community you'll get is a homeless encampment and maybe a taco truck
  
  A lot of these replies are tone deaf. Not everyone lives in a place where walking, biking, or even ride share is viable.

Don't you have public transport? I usually take a bus to a gallery or cafe, and the mechanic phones or texts to say my car is ready. The repair place also has a few chairs and a free coffee machine, so you can sit and wait if you'd prefer. It's noisy but interesting. Maybe they have a break room where you could wait?
- Not everywhere has decent, if any public transport, especially if they're in a more rural area. And not every repair takes less than a day to fix.
- Most places in the US aren't reached by public transit.
- Don't you have public transport?
  No.

when I'm about to have them hundreds of dollars getting my busted-ass, POS car fixed?
As someone who has only ever driven broken down POS, I feel for you OP. Focus on getting it running, not making it perfect. Wishing you well and hoping it's cheap! <3

this is a good example of when it helps to have friends.
- I work near a shop, thankfully. Also, my wife and my parents live nearby. The irl social network is very beneficial.

My shop has a big lounge where you can wait until your car is ready. You can get coffee, tea or water. There is free wifi and I have seen people who brought their laptop to work there. You get a pager to let you know when your car is ready. If you don't stay in the lounge, they send you a text instead.

I feel like this would vary by shop. There's an automotive shop with a relatively small waiting room, because they are less than a block from a ~2 mile long road of shops. Very walkable, just ever so slightly uphill. You can usually walk and find a number of stores to window shop at before heading back.
There's another automotive shop a bit aways from my house. It's on a mostly solitary street, a few miles away from any shops, and is not an area you would want to be walking in. They have a fairly large waiting room.
But I think they all say you're free to to do as you please because there's not really anything for you to do for the next 2 to 48+ hours, depending on what you're getting done.

Do they assume that? I have an appointment scheduled tomorrow and plan to wait in their lobby.
- They prefer that.
  It is one thing if you are getting an oil change or tire rotation, it is another thing entirely when you have a check engine light or an electrical problem that needs diagnosis and there is no way of knowing how long it will take to find out the issue. Even worse is when it is an intermittent issue that the customer can't narrow down to certain conditions.
  Also your appointment will be more like a doctor's appointment if things are busy, you car will be brought in when a qualified technician is available and not immediately after you hand over your keys.