Well I agree with nearly everyone here about life having a high probability of being present elsewhere in the Universe and that they probably haven't visited us.
I was thinking though, we don't actually know anything about life.
We have once instance of life, and that's on Earth, but you can't make accurate observation based on one instance. Imagine you lived in a cave for your whole life, and you met a man with brown hair who was 5 foot 7 inches tall.
This man becomes your friend and stays with you for a year, then he has to leave.
You know a lot about this one man, so therefore you would probably believe every other human to have brown hair and stand 5 foot 7 inches tall just like him, as you don't have any evidence opposing this idea.
What I'm saying is, we don't know shit, so we can't really say it's even probable that life exists elsewhere.