I remember I went to a small showing once as a kid. It was just our group and 1 lady in the theater.
We got to small talk and the lady mentioned she had once been the only customer for a showing and told the projectionist that she didn’t want to be a bother and could come back and another day.
The projectionist had apparently replied that it was no bother - they would roll the movie even if no one showed up!
When I was a kid I wanted to go see The Avengers (the o.g. one, from 1998).
I had to go to the cinema 3 times, because they would not do a projection for less than 5 people.
That must vary by theater, or perhaps practices change from time to time. My brother worked at a movie theater in high school (20 years ago), and the theater he worked would not play movies if nobody had bought a ticket. He told me they would occasionally catch people trying to sneak free movies that way - the projectionist would notice someone in the theater for a time which was going to be canceled, call the box office to confirm if they had sold a ticket, and if not they would get a manager to escort the person out.
Im assuming (though rare) it’s the same with flights. They keep the schedule for movies in case someone joins half hour late. Plenty of people visit my the cinema for all kinds of reasons other than the content (like sleeping in the AC among other things that come to your mind). Keeping the movie going rather than waiting for someone to show up and make it awkward would probably be better for customer service too.