Hi, I would simply say that some expressions, and particularly prepositions, are simply used idiomatically. We say it because we say it, and not with any deep and logical thought. If I really wanted to hunt for a reason, I'd argue that we sometimes talk about the medium eg on film, on TV, on paper, on video tape and sometimes about the content eg in films, in TV shows, in books, in video tapes But I still think it's largely idiomatic.
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