A movie is generally made to speak to its time. It picks up and reflects back to us all sorts of ubiquitous cultural norms of our time.
So what happens when you watch a movie decades after it was made? All of those cultural norms, once so ubiquitous as to be invisible, become glaringly obvious.
An old movie is, inevitably, an anthropological examination of another era. You see the cultural assumptions of another time laid bare.
Although this is never the intention of filmmakers, it becomes inevitably so over time. Maybe this is one of the reasons we love watching old movies.