I am personally asking, because after decades of tech hobbies, I have a bit of a self awareness to 'fun':

I wonder what the fun part was, to them.

There is something rewarding about learning. There is something rewarding about completing things. There is something rewarding about showing other people.

I have the issue that everything I make should be practical. Either net me profit so I can make lots of money. Or useful to society so I can reduce the world's pain and increase pleasure, maybe this is a selfish way to fame.

I still get all 3 of those rewards I previously mentioned, but there is something different going on when I'm doing something for profit/others. Its a different feeling, not better/worse, just different. Better in some ways, worse in others.