I for one am a big fan of getting paid. "Doing a startup" is no trivial task. Just getting the funding in place to provide enough money to pay everyones bills is more than almost any "startup" ever achieves.
This might be an interesting way to ensure that I work with people I'm inspired by. I dig what they're doing.
You can also "start a family" while working for an existing company. People so it all the time on nights and weekends.
See what I did there?
Starting a company isn't the most important thing in the world, especially when you aren't doing anything innovation and when you are just doing it because. Believe it or not, just cause you call your startup "Uber of X" doesn't mean it's innovative and doesn't mean you need to spend time building it.
Not so easy once you have a family, I barely have time to mess with new tools in my home lab, working a second job just sounds taxing (the only reason I even have an LLC is legal shielding for the odd repair / small business IT job I have on the side).