for a few reasons. The 40's and 50's were the golden age of film and originality. The 60's and early 70's were experimental and dark, and political. A new generation came of age in the late 70's in through the 90's, a second golden age and kind of invented the summer blockbuster, made movies fun again, but they were still restricted and tied down by technology. So they still had to really work to get a shot. Travel to locations, endure heat, snow, etc...I think you see that extra effort in films like Jaws, Raiders, Star Wars, ET, First Blood, Dances with Wolves, Glory, Saving Private Ryan, Apollo 13, etc....that you just don't see anymore with advances in CGI.