Hollywood started with adaptations of stage plays and books. There was just a higher percentage of new work to offset that. There were more piles of dreck back then that most people now are unaware of, as well. It wasn’t as though Casablanca-type movies were coming out every week back in the day.
People forget Hollywood is first and foremost a business. Shareholders are more concerned with ROI (return on investment) than with creating art. It’s easier, as dynamicduo says, to go with a proven money maker than to take a risk with something new that the public may not buy. There have been remakes and “franchises” from the 1930s on, but its level of application today is certainly bigger than it was than in the past. Even in the 80s, there weren’t as many of these types of films.
Film is now a mature business. Producers and studio heads today are not the risk-takers, storytellers and showmen of the past. They are bean counters. And they grew up with film and television where the old timers didn’t, so they have a language and a system easy to perpetuate, one that the old-timers created.