While certainly not the norm, there is also Jonathan Blow who made Braid & The Witness, both afaik full time with borrowed money. Though few people have his vision and resolute.

Me, I'm fine with making games on the side for now. But if an idea were to come along which I deeply fall in love with, I can easily imagine quitting my day job (being frugal, always putting on the side helps with that) and just work on something for a few years.