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.
This makes sense if someone bought a ticket and didn't showed up, but what if none was sold? They could just stop selling after a certain time and be sure nobody will be there late.
Well it's very different for flights, they need the plane at the destination so they have to fly it. With movies it's probably just simpler to start the movie than to try to manage the logistics of not starting it, just to save 2 hours on the projection bulb.