Joking aside, a real good reason movies are different than books is because we all read the same thing differently and interpret things in our own way. So usually, we don't technically get a movie of the book, we get a movie of someone's interpretation of a book. And sometimes that leads to drastic changes.
Not to mention the other obvious problems with unfilmable sequences and movie run times. You hear people complain about how some book based films are condensed and leave big things out. Those are films that want to get the meat of the story. Then you turn around and there's stuff like the Hobbit where everyone complains about how long and padded the film is, when it's trying to tell the entire story. Even the parts that shouldn't be filmed.
Though I still suspect they want to catch cheating kids red handed as well.
Not to mention the other obvious problems with unfilmable sequences and movie run times. You hear people complain about how some book based films are condensed and leave big things out. Those are films that want to get the meat of the story. Then you turn around and there's stuff like the Hobbit where everyone complains about how long and padded the film is, when it's trying to tell the entire story. Even the parts that shouldn't be filmed.
Though I still suspect they want to catch cheating kids red handed as well.