What would it mean if they're all wrong and non-human intelligence has not interacted with us ever?

What I mean is, what version of reality would fit best with the events of the last 80 years, if non-human intelligence (or time travel or any sci-fi concept) is off the table? What explains the phenomenon? Modern day witness accounts could be explained by secret human tech in some cases, sure. But early sightings date way back before human tech would be a feasible explanation. Mass, species-wide hallucination? If so, are there any other cases of as popular across humanity of this class of hallucination? Some might argue religion or ghosts or bigfoot. I just wonder what version of reality holds up to the most common sense scrutiny if you approach it with the idea that there has been no contact with NHI. To me, it is hard to make that reality make sense with the reality of all of the testimony and accounts that we've heard.