Former astronaut Alan Bean, who walked on the Moon, believes that aliens haven't come to Earth and there's a reason.
One of reasons I don't believe they have been here is that civilizations that are more advanced are more altruistic and friendly — like Earth, which is better than it used to be — so they would have landed and said 'we come in peace and we know from our studies you have cancer that kills people, we solved that problem 50 years ago, here's the gadget we put on a person's chest that will cure it, we will show you how to make it'.
Bean believes there are alien civilizations, many far more advanced than we are, but is it that they don't think we're ready yet for them to appear?
Look, I've watched Star Trek, I know all about the Prime Directive and all that. But if there's an alien civilization out there that is altruistic, knows how to get here and knows how to cure serious diseases, then they're just being jerks if they haven't shown up yet.