yea if they did the novel's ending in the movie, it would have looked really weird and psychedelic.
NOVEL ENDING SPOILERS
it's been several years but I remember something about their minds going into space and looking at the multiverse. and meeting this other God Being called The Turtle which helps them in the final battle in some way. It honestly would have been kinda cool but I guess I can see why they didn't want to do it. your average movie goer just wants something more easy to understand and basic.
It's even weirder than that.
the turtle is the antithesis/adversary of it, but dies before it can help them. Bill sees its corpse, having suffocated on a new galaxy it puked up. The Ritual of Chud goes by the idea that IT must abide by laws of their world, and they bite down on each others tongue and make each other laugh to defeat it the first time, but it gets away so they don't kill it.
Book ending is not for everyone, much as I love it.