1. They want money
2. They are a bunch of talentless hacks, for the most part, so, like llms, they regurgitate the past with hallucinations of how good original stories can be "improved"
3. People are dumb enough to keep paying at the box office, reinforcing 1
I think of some older movie series like Dirty Harry or Death Wish which were developed over a long time but didn't focus on nostalgia but rather tried to make a really different movie each time. Some later Death Wish movies are silly (Charles Bronson's ability to get weapons in the mail is like Wile E. Coyote) but they're never the same. At least the last three Star Wars movie had a cast that seemed to come right out of Doctor Who.