If we need a common enemy for people to get along then society has already failed

Not necessarily a common enemy, but a common cause.
I've never seen people come together and work harder than when we were trying to end the Y2K problem. I guess you could call the bug "the enemy", but it was a little more abstract than that.
- Were you in an industry affected or are you just reminiscing on being in society
  Because being in society for y2k was utterly meaningless. We did nothing. We did not come together, we did not work hard. A small handful of dweebs noticed the issue and fixed it.
  The USA, with its burgeoning 24 hour news cycle and coming off such hits as oj, princess di, and columbine, recognized they could blow that shit up. So then America wasted 40 billion dollars and a shitload of fear mongering when basically every other country spent almost nothing for essentially the same outcome. Because the outcome was contingent on patches to windows and the Linux kernel, which were obviously going to happen long before 1/1/2000 and regardless of the government because it was a glaring bug that was found
  I do ultimately agree with your sentiment though. A common enemy is not necessary and we absolutely can be unified around a cause. The space race obviously had Russia as a villain. The new deal is something that on paper could unify but in practice saw conservative opposition and liberal criticism that it didn’t go far enough. I still think it’s possible though, even if an example is a challenge to think up
  
  I was a front line worker during Y2K with 60 software updates across 600 systems from Texas to Guam.
  I had to run a color coded Excel sheet to keep it all straight, it was CRAZY.
  In my case, because it was the automotive industry, it also all had to be done early because of the '00 model year cars. Like I say, I've never seen people work harder.
  Even then, there were still problems outside our control:
  https://www.capecodtimes.com/story/news/1999/10/13/y2k-brings-back-horseless-carriages/51026660007/
- And now people are saying climate change is like the Y2K bug and "it will fix itself" 🙄
  
  Those people don't even KNOW about the 2038 bug... ;)
  https://en.m.wikipedia.org/wiki/Year_2038_problem
  Fortunately(?) it's unlikely I'll live to see that one.

Ironically, we do have a common enemy: The billionaires. Problem is the enemy has managed to brainwash almost half the populace into fighting for them.

Truth.
Though, America got by for a long time with that being Russia, and Nazis...
What happened to that
- Whoever fights with monsters should see to it that in the process he does not become a monster.
  
  A more applicable version would be Whoever fights with monsters should see to it that they still have something to do in case they succeed.
- Having the local dictator as the unifying enemy seems to be popular. Just ask anyone from Tunisia, Egypt, Libya or Yemen.
- Well except for all the innocent people swept up in the red scare.
  Or the minorities living during those times.

Crazier idea. Let's create a fake racial group, and blame all our problems on them! We'll first manufacture a fake racial group that no real human could be mistaken for. Then we'll create fake history, fake documentation, maybe even invent an imaginary island they originate from. We'll run endless fake news stories showing them committing horrible crimes. Etc. Turn the hate machine up to 11.
It won't fool the most intelligent, but that's fine. Everyone who wants someone to hate will have someone to hate. And no one will have to suffer for it.
- The mole people.
  Bonus points, we can direct the idiots to start digging for them, then use the excavations to start building underground. It'll massively reduce the power need for AC due to global warming, and it would free up a ton of space for solar.
- This is a conspiracy theory I can get behind
- History repeats itself. This will happen again and again. Well written

Insert "Always has been" meme here

This is the underlying plot of plenty of movies. The prevailing solution is to find something other than each other to be our unifying enemy.
Conclusion: Hopefully we get attacked by aliens or something.
- We had a global pandemic and that didn't work. Maybe an alien invasion would work but I think it might just be a plot device.
- The global pandemic of COVID-19 was not a visible menace, and only endangered the poor, the old and the weak.

As an ex-biologist, I think this is just down to a choice of words: as a society, you could see Nature as an enemy.
Man, even as an individual, or even a monocellular organism, you could argue that entropy is your enemy.
If an enemy is a useful concept for maximizing potential within your scope of choice, then be it.

Bonus point: now look at how old humanity is. The Greek civilization, the Chinese, the Egypt... literally empires have come and gone, yet humans are just as dumb as thousands of years ago

We need something in common for people to get along. Enemies are just very easy things to share between groups, but common creeds, ideals, projects are all unifiers of equal power (though they're not nearly as convenient to find...)
- You'd think being people would be common enough... life has enough difficulty without us creating more for each other.
  
  Unfortunately I've found that unless they're given a very definitive common focus, humans are exceedingly prone to spending their free time carefully cataloging their differences.

We have built within us a need to fight for survival. Natural selection has bred us to constantly be fighting for the top. When we get there we have no idea what to do with it.
We need something to fight against, that's why we all love under dog stories.
- Uhm, no. I am fine with my belly full, a place to sleep and a confidence that within foreseeable future no big bad thing is looming over me or my family - with that settled, I have no need to fight.
  It needs to be said that the above does not mean I am not going to out effort in things beyond that scope, but those efforts come from the desire to make the word a better place, not from some fighting

If society needs to all get along then it has already failed.

You've got it backwards. Society doesn't work without a looming threat, whether that's an enemy or the environment.
- How so?
  Unless your "enemy" is "anything that most people see as a source of suffering/pain/other kind of unpleasantness", I do not see how your statement can be true. And even then it is still a dumb way to exist

No, it means that your society is not homogenous and most of the time people do not feel themselves as part of it.
- Feeling a part of something has nothing to do with homogeneity, it has to do with empowerment.
  What you’re describing frankly sounds like ethnonationalism.
  
  Feeling part of something has a lot to do with homogenity. Most nations are built on ethnonationalism.

Single dumbest fucking thing I've seen all day