Why do they change books to make them movies??

Big Daddy and I recently sat down to watch some new releases. We don't watch movies very much and hardly ever rent them so this was a big occasion. We each got to choose one movie. We have watched my choice so I will proceed to talk about it now. SPOILER ALERT: IF YOU DO NOT WISH TO KNOW ANYTHING ABOUT THE MOVIE THE DEVIL WEARS PRADA THEN YOU NEED TO STOP RIGHT HERE!!!!
A few months ago, this book was on my nightstand. I thoroughly enjoyed reading it and thought that it was sure to make a great movie. Well, I was completely wrong. I suppose that if you are not anal like me, you might like the movie just fine. I don't mind a few changes here and now as I understand that the movie can only be so long, but COME ON PEOPLE. I see no need to basically rewrite the whole story just to make the movie fit your time frame and budget. I thought that the movie made Miranda entirely too personal. The Miranda in the book would have never EVER told Andi something as personal as that she was asked for a divorce. They were much more chummy than they were in the book. Also, the movie took the focus off of Andi's other relationships so that you couldn't see that she was neglecting all aspects of her life and not just her boyfriend. All in all, it is a good movie but I guess I can't separate the book and the movie very well. Are there any other movies that you like but think the book is better??
Tonight we are watching Big Daddy's choice the Oscar winning film The Departed. I will be sure to let you know what I think.