Book to Film adaptation: why they mess it up.

Many of us are familiar with Hollywood's infamous botchery of many classic and popular books. Many times a book may have an excellent storyline yet Hollywood pillages and rapes the story into something that little resembles the original masterpiece. I have three possible theories for this phenomenon.

1. What makes a book's storyline great differs dramatically from what makes a movie's storyline great.
2. The book's storyline may be too difficult to adapt to film as it is.
3. The screenwriters, directors, and/or producers are fucked up.

I have read books that were expertly written and had excellent plots yet when I try to picture it as a film it just sucks. Some books will work while some will not. Think about it.