Why do so many investors choose to keep their day job?

[Billy Rogers]

I've noticed that a lot of investors keep their day jobs after they seem to be very successful. I've met people who have been flipping houses for over ten years and still have a day job. I'm too polite to ask them in person why they still have a day job. I don't mean this to sound negative or rude, but why do they do that? Are they not making enough to quit or is it some other reason.