Because I would rather watch something I've never seen before?
I've heard a story on Reddit of a guy watching Big Trouble in Little China everyday for years, but usually people wanna watch new stuff. I guess people also rewatch shows like Friends and The Office, but do they really "watch" it or do they just play it as background noise?
Consider that new doesn’t mean good .
Even if you just like old movies, there are tonnes of great old movies. Don't you want to watch more than one?
Experiencing something for the first time just isn't the same.
Learning how to continue appreciate things that you've already experienced gives you the power over the experience. You don't have to depend on an a constant influx of "new" content.
I've seen Interstellar, Matrix, Gladiator, Harry Potter, Lord of the Rings 20+ times. They are even more moving now when I watch them because I can see how I've changed and related to the movies in a new way.
I really don't care for it though. I have 0 interest in watching Harry Potter 20 times. And considering new content comes out over 10x faster than I can consume it, I don't see "depending on new content" as being a problem.
It's impossible to catch everything in a well made film on first viewing. Not everything is worth watching more than once but for me the first rewatch of a good movie or show is actually much better than the first viewing.
I like knowing what is going to happen, and having the freedom to let my attention wander to aspects of the media besides the primary plot and dialog, and keeping up with what is happening.
Not everything is worth watching twenty times but anything good is at least worth two viewings. The very best media has layers that unfold each time you experience it.
And tbh, I feel like movies are very meta and you also get more from each movie if you watch more movies/read more books. Like Doctor Sleep for example, is an adaptation of a Doctor Sleep novel, but also a sequel to both the Shining novels and movies. You would get more of it if you read the two novels and watched the shining movie than if you just watched the movie or read one of the novel twice. I also enjoy watching different adaptations of something and pit them against each other, like Let the Right In (both the novel and the movie) and the American adaptation. I guess that's my version of what you get when you watch a movie twice.