I'm going to put my feet in the shoes of the Aliens:
I'm a part of a race of species millions of years old, or old enough/smart enough to advance technology enough to travel through space faster then the speed of light.
Why would I come to Earth and not:
-Show myself to everyone, and not be a secret.
-Teach Humans our technology so that they may join us in inter galaxy trade and meet the rest of the species from around the galaxy, and fix the growing environmental problem Earth has.
-Conquer everyone and harvest the planet for resources.
Instead, we come all this way and agree to accomplish nothing but remain a secret and neither race gains from millions of years of developed space travel.
--
Too imply there are Aliens that have visited Earth is too imply that,
A: They weren't smart enough to realize they would die in our amostsphere and every time they tried we got lucky and a government official found the space pod before a regular person.
B: They struck a deal with our high ranking officials.. in America or, leaders all over the world? to agree to keep themselves a secret from regular Earth people at what gain, and somehow anyone with proof which could be a lot of people is able to keep it secret despite it being the biggest secret the world has ever known.
I dont know, I dont buy it