This isn't only a problem with prequels, but with sequels and reboots as well. The lack of originality in so many movies, much of which are made by studios like Di$ney, is baffling.
Yes, sometimes a sequel / prequel can be absolutely necessary, however, like you said dk, there are a great many films that would be better as standalones, like (in my opinion) A Quiet Place (but skrew that I guess).
Audiences nowadays like to be spoon-fed, constantly wanting more, because what they have now just isn't enough for them, and they have a problem with speculating or leaving unknown plot points to their interpretation... and studios give into it.
It's obvious that these studios see this as an opportunity to make tons of money, and they forget to come up with completely original ideas. A big problem is that people a lot of the times forget to support actual good and orginal films, instead focusing only on big blockbuster universes like the MCU, which is a shame.
"What’s so great about discovery? It’s a violent, penetrative act that scars what it explores. What you call discovery, I call the rape of the natural world."
— Dr. Ian Malcolm, Jurassic Park (1993)