Maybe Lucas and Spielberg accidentally destroyed the future of US big studio cinema by making such profits that it attracted multi national corporations to buy the studios. I think that led to the eventual decline of the US film industry--and when the old time studio production executives retired and were replaced by business school graduates --voila--Transformers 2007/Clash of the Titans 2010.
On the subject of 50s cinema--the thing is that those films were considered kiddie fare and relatively cheaply made with weaker screenplays and acting. A big studio wouldnt have touched it. Columbia was formerly poverty row after all.
Jaws onward were big studio films with large budgets. The studios put all their resources into the "kiddie fare"--at least that's the perception.
No doubt the success of Star Wars and Raiders helped kill the more serious vein of science fiction the big studios were doing---the 2001-Planet of the Apes-Soylent Green, Silent Running variety...that's too bad.
The rising cost of filmmaking is also a big factor too obviously.
Less risk taking.
I think the success of the prequels just from brand name recognition probably encouraged the studios to focus more on franchises and dispense with original screenplay projects. Why bother when people (seemingly) want another Batman, or will watch a movie made from a Disneyland ride.
The corporate takeover of book publishers has also had a dire effect on storytelling. Feels to me like there is less variety, less books being turned into films.Would even something like Jaws be possible today given that they only care about recognizable franchises? Are there best selling novels anymore that arent about a boy wizard? lol