> really curious if any of the downvoters could mention their rationale.

Not downvoting, but I used to be a huge "UFO fan" around the ages of 8-11. I read book after book, all breathlessly outlining the "reams of evidence" available. I watched documentaries on television, saw UFO topics covered in the newspaper, and I had a shelf full of books all in agreement. Must be true, right?

Except that even as young child I started noticing that all of the photos were blurry. All of them. Focus dispels UFOs just like turning on the light dispels the monster in the bedroom.[0]

Years later I read a book by a "UFO fan" who remained a fan into his adulthood, and got the opportunity to research them as his day job. He was looking into crop circles and cattle mutilation specifically around the time of their peak popularity[1] but as a result of his investigations he rapidly converted from a life-long believer to a sceptic.

Why?

Because he noticed that that evidence of UFO visitations respected state borders. Specifically, there are state-level laws[2] in the USA related to things such as insurance claims related to dead livestock. Cows are stupid, eat poisonous or dangerous things all the time and die. One dead cow represent a loss of hundreds to thousands of dollars. Many dead cows could be a serious financial problem, so there is insurance available for lifestock. The policies apply differently in different jurisdictions, and some would cover "unexplained external causes" such as little green men anally probing cattle for mysterious reasons, and in some areas the policies would not cover this. Unsurprisingly, cattle mutilations and the associated evidence like crop circles would only turn up in areas where the insurance covered it, and never in areas where it wouldn't, even if that was across the road in a paddock owned by the same farmer. Odd huh?

One theory -- that sells books -- is that little green or grey men visit our planet across interstellar distances and amuse themselves by cutting holes into cattle. But not low-value livestock like chickens. Just the high-value ones, like cattle.

The other theory is that selling books and making insurance claims is the only reason anyone talks about any of this seriously. That people see a dozen dead cows, have nothing they can legitimately put on an insurance claim, and are staring down the barrel of financial ruin. What to do? Just drive the tractor in circles over still green crops, bending them down, call Janice from the local news, claim that the circle is impossible, and point at the dead cows you cut a few times with a sharp box cutter a few hours before the news crew turned up. Suddenly, there's "evidence" that you can put on an insurance claim and your farm will survive until the next year.

> many official and declassified sources

The word "declassified" makes UFO fans excited because it's got all the elements of an official secret that they uncovered through their intelligence and sleuthing. It's the same addictive narrative that made QAnon popular.

Most (all?) military observations of potential enemy aircraft are classified! That means nothing. The value of these observations isn't particularly higher than anyone else's either. If one young pilot sees a splotch on the IR feed that "moves oddly", people run off with that and claim that "government has evidence of UFOs!" This recent one is the best example: https://www.youtube.com/watch?v=rO_M0hLlJ-Q

At first blush that looks exactly like an optical effect such as Glory [3] that appears to move only because the observer is moving with respect to nearby clouds. The background is moving, and Glory remains stationary relative to the observer because the Sun doesn't move in the sky like the clouds do. It "accelerates away" because the IR camera is on a gimbal on the tip of a Javeline missile and reached its limit. It stops tracking the "target" which then seems to "shoot away" in the picture.

This and similar "evidence" is about as good as it gets. I've never seen anything even remotely convincing. Nonetheless, book after book just collates and rehashes the same evidence, including pictures long since discredited[0] as clear fakes. The authors get paid and can feed their families. The readers get to be entertained just like I was when I was a kid.

Everyone gets something, but we don't get to invite visiting alien dignitaries to speak at a UN convention in much the same way that Air Traffic Control doesn't schedule flights differently on Christmas Eve to avoid hitting Santa.

[0] Several famous UFO photos that adorned book covers published by legitimate print houses turned out to be chandeliers that the photographer had thrown into the sky like a frisbee and then quickly snapped an out-of-focus picture of. On commission for the print house.

[1] Speaking of which, isn't that odd all by itself? Crop circles weren't a "thing" until they were. And then vanished again. While they were popular, like a meme on Reddit, there were all sorts of interesting variations. Similarly, the aliens themselves evolve just like bad Fifty Shades of Grey fan art. Some traits are never mentioned by any "witnesses" until after a particular story, and then it's a common trait many people claim to have seen.

[2] I read this over 15 years ago so I might be confusing a legal boundary with a corporate "coverage area" boundary in a contract, but the gist of the story is the same either way.

[3] https://en.wikipedia.org/wiki/Glory_(optical_phenomenon)