I think writing a book to make money is itself pretty naive. There's already a bit of privilege involved in being able to devote time to writing, and in many cases make attempts for years before getting published to modest revenue.
A lot of people start writing books despite knowing those odds and outcomes.
And how many give up after one (if that) when they realize they can't afford the time to do it without enough compensation to reduce hours/replace their day job? Especially when it comes to the boring business side of getting it published and marketed. I can imagine someone writing in their spare time (like any other hobby), but it's much harder to imagine going through all the rest of the process unless they're (a) hoping to make a bit of a living from writing (b) doing it for ego reasons (I guess that's were vanity presses come in)
Self publishing is on the rise. Some people feel the need to share, more than the need to make money... that's why a lot of books get written (I believe), generally not to make money, but to share something (the author feels is) important.