Because it’s insanely inconsiderate and extremely egotistical. You aren’t the only person in the world. We’re both watching the movie and we’re both equally as confused. When you ask those questions what you’re doing is interrupting the movie for someone else to demand an explanation for yourself. An explanation you will not get as, again, both of us are confused. Other people are watching the movie too. Interrupting it to try and get answers for yourself is some narcissistic behavior, especially when you’re in a movie theater with shitloads of other people around you.
Moreover, during the middle of the movie is not the place to have a discussion. People are trying to watch it and enjoy it, not listen to some breakdown about the events so far. If you want to have those discussions then keep it for after the movie when attention can be properly given to the discussion, not a half whispered response in a tone of “Dude, can you shut up?”