Personally, I think it's because people see writing (and other creative arts) as Not Real Work. It's Art, meaning something you should do because you're inspired and passionate about it, which is partly true but kind of misses out on the fact that you can't buy groceries with inspiration and passion. If artists can't pay their bills and buy what they need to live, they're not going to have the time to make Art.

It's highly illogical as well. You wouldn't go to the mechanic and expect them to replace your car transmission because they love what they do; you'd pay them because you know it's hard work that you cannot/will not do yourself. Yet the very same people who read books don't view the creation of those books in the same way.