This is certainly the requirement of a screenplay as opposed to a book, every scene has to be the bare minimum to carry the movie forward. Movies are generally 30-40 3-4 minute scenes so you have to put punch in each scene as opposed to a book which can have a lot of 'description' in it. Many books are passed over as stories viable for a screenplay for this reason. Or as you note the book is whittled down to an absolute skeleton of the story with just the most dramatic bits kept in. Hollywood receives about 100,000 screenplays a year and they make maybe 300 mainstream movies. Economy of words is required. I haven't read the book and have not seen the first It movie so I'll have to go in cold!