Some movies are timeless. But even those can't help but be influenced just a little bit by the time period in which they are made. A film doesn't have to reference an event or a person that future audience may not understand in order to be a bit dated.
Over time, our values and what we accept and societal norms change. People that were historically disenfranchised slowly gain a voice in social discourse, and attitudes around certain topics shift along with that. As these things good things happen, jokes and movie plots that audiences may have not paid attention to or may have even laughed at now make us cringe when we give our old favorites a rewatch. These movies are ones that we noticed have not aged gracefully since their release.