There's a lot of grey area between "startup" and "side project". I do know quite a few people who operate niche cloud services that pull in a few thousand dollars a month. Certainly not enough to build a company around, but the $ per hour works out pretty well. I could see how someone might want to quit their job to work on turning their side project into a startup.

That said, nobody I know over 30 who works in tech does their "side projects" in tech anymore. We've all moved on to kids or hobbies that allow us to escape the tech world like music or painting.