The reason this is 80% of your job is because you will spend the majority of your time fixing shit.
And you will do it with a smile, because it’s your job to make it all come together like it was meant to be that way from the beginning and not be a princess about it because the story you pitched in Pre-Production and which everybody liked is now half the size and parts of it don’t really make sense and let’s not even talk about that ending level and someone’s going to write a blog about how you’re a crappy writer because who would ever write a story like that intentionally? If any of that’s a problem for you, then go write a novel and be happy.