Movie franchises fritter out because because most sequels weren't planned and are just rehashes of the first movie in the hope that they can recapture its audience. They're happy to piss over existing lore (breaking up the main couple, killing off beloved characters, destroying what the heroes were trying to accomplish in the first movie), if it makes it easier to hash out a script while the franchises' window of popularity still exists. Big movie franchises also tend to be spectacles, filmed around special effect/action setpieces. The story is often just an excuse to tie together whatever elements currently serving as a dopamine drip for the audience.
Seriallized storytelling works better in TV, especially now that cable/streaming is a thing and networks no longer have to worry about making each episode a stand-alone story, potentially airable in any order. I'm certain anime, which has been telling long form stories for decades, had something to do with making this popular in the West, as more and more nerds started watching it. Nerds will often bash older fantasy movies for being somewhat on the nose about their exposition, using narration or an exposition dump to relate story to the audience, because they're so used to long form TV or movie series that unfold the plot slowly over several episodes (or over several movies.) While exposition dumps can be done badly, that's not always the case. And some exposition dumps
can even end up being the highlight of the movie they're in. In the rush to push more profitable long form storytelling, short-form storytelling often gets dismissed as frivolous or cheap, even though it can often be of a higher quality, and be far more accessible to casual audiences, than long form storytelling.
There's something to be said for watching an epic fantasy story unfold over an entire season or series of movies, but there's also advantages to having a tightly scripted story that tells an epic story in 90 minutes or less. Movies used to be able to do that.
Big Trouble in Little China -one of the most complex and action packed fantasy/action movies I've ever seen, wrapped up it's story in a brisk 99 minutes. Not a single minute of it feels wasted or rushed. Nowadays, you can't make a special effects laden genre film without padding it out to 2-1/2 hours or longer, otherwise audiences will feel cheated. America has done with movies what it has done with restaurant food -giving patrons over-sized portions of bland product that are barely consumable in a single setting, because people complain if they get any less than that.