Don't know.
I see UFOs all the time - but thats because I'm not very in to ornithology.
The chances of there not being extraterrestrial life seems laughably slim in such a large place as universe.
But ETs travelling between stars and visiting Terra (and without properly saying 'hi') seems a more tenuous assumption.
On the other hand, something is going on here: People do see them. Sometimes en masse. People have had strange experiences and some tell of being abducted. And this sort of thing - whatever it "is" - appears to have been going on throughout a lot of history. But whether it's down to little green (or grey) men, the elves, hallucinations of some sort, brain tumors, or some other explanation - "theory X" - is a much murkier point.
Hoaxes further muddy the waters, but the pure skeptical "it's all a hoax"/"it's all a hallucination" explanations of individual often gets so contrived and convoluted, they end up looking less plausible than the "ETs did it" theories.
Read Charles Fort's "Book of the Damned", and you find yourself entertaining the opinion that the universe is not only weirder than you imagined, it's weirder than you can possibly imagine.
I haven't really come across an explanation that seems to fit yet, so I remain a model agnostic.