big shots enjoy making movies (or at least, they keep doing it; who knows if they enjoy it). but scripts take a lot of time to write, and sometimes it saves time to simply take a pre-existing work of fiction and modify it for the big screen. however, sometimes the source material does not pass through such modification unscathed.
the movie-fication process often takes pains to shorten, lengthen, doll up, ugly up, or in other ways mutilate the source material. e.g.:
* apocalypse now: the original book, 'heart of darkness,' is set in 19th-century Africa instead of vietnam.
* a clockwork orange: alex is cured of his violent tendencies in the end
* forrest gump: gump goes into space with a monkey and a porn star
* jurassic park: the lawyer wasn't a bad guy but hammond was
* starship troopers: the movie and the book both examine the message of "the army is way cool, and better soldiers make better citizens." the book honestly expects the reader to agree with this message, while the movie is laughing in your face for thinking it for even a second.
* who framed roger rabbit?: is about comic strip characters, not animated cartoons, and the killer is like a magic genie or something.
* every stephen king book: doesn't suck. well, not all of them.