Because a film or television show, regardless of the quality, is to be experienced with full focus. You aren’t supposed to wander off or daydream when watching something. You are supposed to take in the characters, dialogue, cinematography, animation, etc., take in the story, and follow it. Escape into that world, not focus on anything else. That’s how you watch something.

You are implying that they are rude because they have ADHD

I didn’t say they were ‘rude.’ Them not paying attention to a movie does not affect anyone else. I’m not saying that’s ‘rude.’ I’m saying not paying full attention to a movie, a play, a show, or whatever is a waste of time and no way to watch a film.