Adapting a book into a movie is no picnic. You’ve got to think about 1) the people who haven’t read the book and are just looking to have a fun time, as well as 2) the people who HAVE read the book, and love it, and will wish death upon you and your family’s family if you get anything even slightly wrong.
It’s a lot of pressure. I have this theory that some people deal with the pressure by just not reading the book, full stop. After all, you can’t mess up the adaptation if you don’t bother to read the source material. Here, I’m pretty sure, are some examples.