Because you're not actually there for the movie but to spend time with the other person. Theaters are different but then again, there's a reason it's a shit way to hangout or go on a date
If you go to the theater to watch a movie then you need to sit down, shut up, and just enjoy the movie. Nobody needs to hear your thoughts in that exact moment. That includes yourself.
So we can share quiet, non-intrustive reactions and absolutely gush about the movie at the end. The hangout doesn't end just because the movie's over. Do you try to talk to the person next to you about the rollercoaster you're having fun on?
To share the emotional experiences, and then talk about the story afterwards.
Or you can talk during the film and have this conversation later:
“What was that dude so angry about?”
“I don’t know. We were talking about the previous scene during that scene.”
“That movie sucked. It was confusing and I just couldn’t get into it.”