Occam's chainsaw
Nobody has ever managed to explain why aliens, after spending inordinate amounts of resources, fuel and time to come here, would just play hide and seek - for decades. Why, for crying out loud? I could understand them being here once, for a short period (it's an exploration mission trying not to disturb the wildlife), but not for half a century.
Also there aren't any two similar UFO descriptions. What is this? The Intergalactic Spaceship Show? If explorers (invaders, harvesters) came all the way to our backwater, they would only have one (or at most two) different types of vessels, that's all. But here we have dozens of wildly different shapes and flight characteristics, and the only thing they have in common is they look "different" and can do "impossible" things.
IMHO people did see something, but definitely not aliens. When I was a kid I saw a "UFO" too, actually in my case it was a meteor. I knew because I had some education, but I can imagine some uneducated kid desperately trying to explain what this extraordinary phenomenon might be, until he is offered the convenient explanation of UFOs... Even earlier in time it would had been a sign of [local deity], ordering you to [do zealot stuff]. *shrug*
Biting the hand that feeds IT
