is one of the sayings that I've put up on Summary Justice because it's often said as though books are an intrinsically superior art form. In a way this might be true, different art forms lend themselves to different forms of expression and I think too many movies try to do what books are better at. Books allow greater depth complexity and a better platform to understand people's subjective experiences. But film is better suited to other stuff; physical stuff performed fast and with great skill ("unconsciously") is much better on film, which is why the Bourne films are so good (I haven't read the books). Also, movies are a visual art; we don't (just) want to read a book explaining the Last Supper, we want to look at the painting.
But, despite my irritation at the saying, there is actually a good reason why the book should, on average, be better than the movie and that's because the book almost always comes first. It makes sense that people only make movies of good books; books that have already been successful. So there is a selection effect, the book can't fail but the movie can and half the time it will.