I think this is because movies are permanent. What happened in them happened and therefore if you were in it, that's it, you were in it. But a show has different episodes and seasons and when someone appears they probably won't be 'in' every single one. Like guest stars on Sesame Street. Katy Perry was 'on' Sesame Street.
I think because when you're talking about a movie you're just talking about one movies... Whereas when you are talking about a TV show there's multiple episodes.
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