Many of your favorite movies were adapted from books you never read. And sometimes that's a good thing. People who have both read the book and seen the movie often say the book was better, because the book usually lasts longer and has more detail. That's fine for a great story, but sometimes movie directors and scriptwriters see room for improvement and take things to a more satisfying level.
Now, let's be fair. Mean Girls was written by Tina Fey, who was inspired by the book Queen Bees and Wannabes. The plot is all Fey's.
I wouldn't even want to read that book. See 30 examples of movies that improved upon its source material at Cracked.
When they go to make movies out of Kurt Vonnegut or Ray Bradbury books it's very hard, for obvious reasons, and you can give them a pass because they tried and you can manage to enjoy it. But The Princess Bride was a brilliant book practically written for film by a pro screenplay writer who also wrote the screenplay; it's impossible to screw it up, so of course they did.
Rob Reiner made some very good movies in his time, Bert Rigby, You're A Fool, for example. I don't know what happened with Princess Bride; everyone involved in the movie seemed to think things went well. Maybe it's time to try again --except now they'd probably use uncanny-valley CGI puppet people and put all the money into rendering the characters' hair.