It seems to me like humans have a basic desire/need to see their own creativity come to life and to truly take ownership over that creation without other people meddling. This human need largely goes unserved in our economic system which, in my mind, explains why someone would opt to work on open source software on a weekend writing more code, after having spent a 40-hour-workweek having written code, or to write a book about marketing, after having spent a 40-hour-workweek having worked in marketing, etc.
So, if you are going to go to the trouble of writing a novel, how on earth would it occur to you to not avail yourself of the opportunity to satisfy that need?