Because humans:
To elaborate: It IS really hard to get into making movies. That's why artists are always looking to make a movie at all costs - because once you're in, you have a reputation. Because of the money involved, most folks don't want to take a chance on an unknown - even if they love the stuff. So many movies are based on stuff that's all ready successful (like TV shows, comics, or books) precisely for this reason.
Some of it can be translation. I loved the book The Postman, but the movie was REALLY terrible. Different formats can fall flat describing things originally in a different media. Editing can be a problem since a longer movie might have tied those elements together like they wanted to, yet there wasn't time to put it all into 90 minutes less credits.
But people are also lazy, and want success to be easy. So once you're in, you are assumed to be good until you make a mistake. So the inside talent pool is small once a movie is picked. If your works make money despite a few flaws, no one cares.
My favorite "skit" from Family Guy was Stephen King sitting anguished at a typewriter. His editor asks him what his next work is about, and in desperation, Stephen grabs a lamp and says, "It's about a lamp monster!" The editor replies, "Are you even trying any more? (sigh) When can you get it to me?"
Sadly, I might watch the Lamp Monster movie.
Then there's the fact that a lot of viewers don't care about a few obvious flaws if the movie is fun. Anyone who's watched any Smokin' Aces films, John Wick films, or any number of others can see there are glaring deficiencies, yet it's fun to watch. Sometimes people just want to suspend disbelief in the process, not just the story. Give folks a great gun fight or space battle, and they're happy. After all, movies are a visual media and great visuals are as important as a good story.
Also sadly, I've watched both Smokin' Aces movies and all the John Wick movies. If they come out with another Star Wars movie, I'll probably watch it as soon as I'm done watching Solo (sigh).