No one

I work with several such people in my department. I prefer not to reveal much information about my work but I'll just say it's with electronics.

Three of them are retired and one of them works for us in his spare time when he's not traveling across the country for his "main" job. The later one offered to do the work for free because all he really wanted out of the job was to learn how the work is done and how he can do the same work for his retired father.

Of the other three one recently had a second "retirement" because he no longer had the spare time and now cares for his wife full time. Another one recently had an argument with management because he wasn't logging all of his hours on the job while working from home and would regularly do additional work that he wasn't being paid for.

I've also had one prior side gig type of job where I know some people did it for fun while getting some cash on the side in the entertainment industry and another job with a similar situation in sports.

I'm not going to pretend much of this is very normal by current standards and I'm certain the majority of people involved were also motivated by having at least an additional source of income, but there absolutely are people who even within our current economic system go to work for the fun of it. But the only ones I've ever met who do it are people who no longer need to worry about making ends meet.