That's a good question...
I'm not sure where I stand on it yet, but to argue the other side:
Maybe because a painter doesn't pay to have his painting duplicated into 1000's of copies to distribute and let the world see?
Maybe because a movie you distribute generally costs exponentially more than it costs to write a book, so you have to recoup costs? Sure a book takes time to write, but it's generally considered "free time" and only that of the author, not an entire crew.
Also, maybe because movies aren't solo projects? No matter how many hats you wear, there's always someone else involved at some point so it's more of a "team effort" than a manuscript.
Again, just throwing some arguments out there. There's only a few books I've seen out there that IMO seem to serve as an EGO boost to the author and nothing more. I tend to think the best (or try to anyway) of most people though.
Good post!