I know multiple software devs that play hockey 3 times a week during the work day. They could easily be working 2 full time jobs at the same time, but why would you, if you didn’t need to?
I've considered a second job as well, but I'd have to skip out on my noon Jiu Jitsu classes and freedom to play Pickleball whenever I please. I'm paid well enough where the value my hobbies bring outweighs the extra income of a second job.
To be trash-philosophical about it, sometimes we forget the aim of life isn't to get a particular number (the amount of money in our bank account) as high as it can be, like it's a video game score...
A significant number of these folks appear to be FIRE focused though. It's an interesting idea, sacrifice your 20s to live the rest of your life work free.