Why do some developers basically work for free in open source projects?

The best coders spend lots of unhealthy amount of time in basements isolated. Github stars and pull requests is the simple vanity that makes them human like girls with big nyash posts. The good thing about this is that society benefits from free code that works.

With AI all those git badges and Stars won't matter anymore since anyone now can inflate easily with solid code.

Passion and donations.

Passion, and giving back to society. Many of them have made good money in career or business.

So you can put it on your CV

Or you just like the idea behind the project so much

Such work would be very insane, unless they own the project - not mentioning tht most of these projects do have a "Codes of conduct" with statements like "Be X", "Never do X there"
So they don't only work for free, but also like being ordered around while doing unpaid work & are fine with that. There are also projects which give their contributors humiliating names (Linux "Kernel Janitors"?) and they are fine with that either
THen I would guess, reasons are likely:
1) Lack of self-esteem
2) Insanity or demonic posession (how insane would you need to be to do unpaid work?)
3) Devotion to projects owners (same as #2?)

Though it's different in case they own the project/part of it. Then it might be for
1) Building portfolio
2) Donations
3) Personal fulfillment
Then such work is perfectly fine & likeley very creative

Multiple reasons. Firstly, it looks really good in your resume. Kinda gives you a competitive advantage. Secondly, programmes like GSOC gives you paid internship offers (they pay really good). Lastly, many people have got very high paying jobs from open source companies they have contributed to.

Just for fun. If I can maintain interesting project why not.

because people are god damn bored and lazy to do their actual work ffs

For their CV