I don't mean to be a pedant, but you mean when you say 'UFOs' you mean extra-terrestrial visitors, don't you? It's an important distinction because even if you assume any given UFO sighting was not just a hallucination or misinterpretation but something real and unprecedented, assuming it's aliens from space is still a wild leap. A leap people tend to make because we're so primed to think "aliens" when we hear "UFO".