Ya gotta admit, these aliens are crafty fekkers: they've been coming and going for a few hundred years - or more - and, like the most model hiker, never leave any tangible trace of their presence. Not only that, but they've chosen as their favourite stomping ground the US of A, so the rest of us humans can dismiss any sightings with a "yeah, that's 'Mericans for you".
I could quite easily accept that we share our universe with other intelligent life forms, and they may even have come to visit this planet in times past. There are a few unexplained leaps in human technological competence around five thousand years ago, and some oblique references to extra-ordinary if not extra-terrestrial beings written into our legends from that time too.
But I think it's far more likely that the overwhelming pollution of the north American continent in the last hundred years (with synthetic chemicals and unnatural levels of nuclear and electromagnetic radiation) has been triggering a set of relatively predictable, repeatable reactions in the human brain - an organ that we know to be very sensitive to such stimuli.
I'm no expert but a quick search revealed the longest running ufo investigation is from GEIPAN which is headed by the French Space Agency.
They're actually spotted all over the world, not just USA, I could list a bunch of other countries with sightings.
Our government currently is the one being active about this because we have the best radar systems and all our jet pilots see them on their radar all the time. I completely reject your notion that I am seeing things because of pollution lol. I have no history of ever seeing anything that wasn't there, I wasn't hallucinating