Simple, YOU help "write" it, or at least "fill out the blanks". No matter how well an author describes the characters and surroundings of a book, their image will always be assembled by you, in your mind. Whereas in movies, well, you already get the picture.
Then again you also have a more recent trend, where they publish books written after blockbuster movies. And, of course, you find the sorry fucks like me, who'd take a movie over a book any time of day, since that'd save time. So nice that we're all different.
