Anime, cartoons, tv shows, and video games are more accessible and easier to digest in a single sitting.
Wait, no, no, no, no. You got it backward. It was the other way around.
You see, cinema in general is pretty neat: 2-3 hours of story in compact size, with neat plot and ending. And that's why, neat plot and ending, combined with focus on single/ small number of issues (that usually get resolved in the end), ensures that few people will think about it more thoroughly. No imagination, no story.
Longer piece of works also help breeds ideas, since it will help to make us aware that there's a big, unexplored world, with all the interesting stories happened in the background where the writer (usually) not bothered to put much scenes on it. Perfect for your OC.
there's also more downtime.
if people are watching a standard 26-episode series, that's 25 weeks of not getting a conclusion and letting your imagination go wild.
a movie you might get a few minutes to think about it if you pause and go to the bathroom. then you get the conclusion and it's all over.
For me, part of it is getting to know the characters better. I find it easier to write about characters I've spent, say, 16 hours with than only a hundred minutes.
Got a degree in Emotional trauma via fictional characters aka creative writing. hosting S'mores party in Hell for fellow (evil) writers
Fanfiction mostly happens to ongoing creative projects, because those have the persistent interest. Without the episodic format, by the time people finish writing their 60000 word continuation story or what have you, everyone who was interested has seen the movie, discussed it with their friends and moved onto the next thing. Note that far and away the biggest movie fanfic worlds are for movie *series*.
At least compared to other mediums. Movies get a tiny amount of fanfiction compared to anime, cartoons, tv shows, and video games. Why do you think fanfic writers aren't as fond towards movies?