Some things are easier to do as a side job than as a main job if you have enough money to live on to start with. Any amount of variability or risk is automatically cushioned until you hit profitability.

Or not. I've seen idiots play big business boy before and come out way negative and they keep going. It's not a side hustle for them, it's a hobby pretending to be a side hustle the way they do it.

One could ask the same about billionaires

it's a hobby for a lot of people