Sometimes, filmmakers leave stuff out of the movie because there simply isn’t enough time to include everything that happened in the book.
But other times, filmmakers leave stuff out of the movie because the stuff they’re leaving out is WAY TOO DISTURBING TO FOIST UPON AN UNSUSPECTING MOVIEGOING POPULACE. Who wrote these books, even? Sociopaths? (Probably. Most writers kill off way too many beloved characters NOT to be.)