Why do so many adaptations suck?

I read the book, and loved it. I don't understand why movies make meaningless changes. I mean, how do you just leave out his son? I think I liked it until about 15 minutes in, but then they changed so much that I think they should have just changed the title of the movie like they were going to so people who actually read the book wouldn't waste their time with the movie.

Just a couple of other points though,
- I think how they filmed the prison scenes was good, I wish they had a bit more of the scenes in the middle of the movie like in the book.
- When I was reading about him dumping the body, for some reason The Machinist came to mind, and I think Brad Anderson would have done a better job with the movie.
- I know that they changed scenes because they don't want a long movie, but I think if they kept the plot closer to the book, it would keep viewers interested longer than just under 2 hours.