Probably because they have lost their skill.
We all love bad movies.
If they’d lost their skill there’d be a change. There isn’t. Since the beginning of the movie industry the majority of all movies have been based on pre-existing material (books, plays, previous movies). It’s got nothing to do with skill and everything to do with cost. Until the modern age of pro-sports movies were the most expensive entertainment to put out, they have all the costs of plays plus the film costs and reproduction costs. Since day 1 nobody has wanted to spend that kind of money without some sort of “guaranteed” audience, and the best path to a “guaranteed” audience is to do something that already has one. So the film things that already built an audience, including remakes of movies and sequels.
There’s still tons of great movies being made, the skill is there. Nothing about the situation has changed for over 100 years, since before Hollywood was actually in Hollywood, conversions and remakes have ruled the industry.