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
Makes for a bit of a boring hangout if you both just sit silently there through the movie. At that point might as well watch it separately and then get together to discuss it.
Cinema being different of course, you’re not supposed to talk there to begin with