Books to movies

It seems like every time we turn around, there is a brand new movie coming out that has been adapted for screen from books. Three very famous ones are the Twilight series, the Harry Potter series, and the Chronicles of Narnia. Having read two of these three series, or most of the series at least, and having seen the movies, I always catch myself wondering why are the movies so drastically different from the books?
My first example is "Twilight." Yes, I was the one watching it critiquing everything that was different about the movie. First off, Waylon Forge, who is Bella's father's best friend, is not even in the books. In the movie, he is a man who is killed by the vampires that come through Forks, yet he wasn't even a character in the books. The thing that made me the most mad, is when Bella visits Edward's home for the first time, and he throws her on his back, flies out the window and climbs a tree. That is so not even in the books, and yet it is a huge part of the movie. He could run extremely fast in the books, which was quite poorly done in the movie in my opinion, but he never could climb a tree!! Why on Earth is that in the movie???
Another example is the "Harry Potter" series. The books have so much detail, so many little things that help you fully understand the story, that there is just none of in the movies. I read the first three novels, but never could get into the fourth due to other reasons, and never got back to reading them. But even while watching the fourth-sixth movies, I had to have an explanation for a lot of things in the movies that I did not get since I had not read the books. They did not bother to include all this back-story information in movies. I just do not understand how they could leave so much out of these movies that were such important things in the books.
Another series currently being adapted to movies is the "Chronicles of Narnia" series. While I have not read the book, or seen the second movie through all the way, I still understand that I am probably not getting all of the story and information I need to fully grasp the whole saga of what is going on.
Just from watching other books to movies, such as the ones listed above, I find that I probably should read the books before I go see the movie. It would let me know more what is going on, and it would give allow me to see those things that were not of the author of the book, such as Edward climbing trees, but were from the screenwriters minds. I have always been of the persuasion that if I read the book and go see the movie, I want to see exactly the same stuff. I be able to watching the whole book in movie format. I understand that others are not like this, but at least the screenwriters should try to get the movie at least a little bit as good as the book was.