Well, I'm just going to add the edge case of close-to-scifi speculation, could it just be that all the evidence of the YD is just from some orbital bombardment? What could have taken to a proto-civilization, let's say, 20.000 years ago, to go from building piramids to build rockets and nuclear weapons, then just nuke everything from low orbit.
Then the survivors restarted from almost nothing (building most of the crappy - but suspiciosly similar to very modern architecture - stone based stuff we keep finding), remember, not every person in an advacent civilization knows how to produce advanced technology. i.e. we have TSMC at hand, but I suspect there are other similar examples.
One reason aliens are not contacting humans could be that we are not actually that advanced. And contacting us would intrinsecally give us sudden acknowledge of some level of technology or theories not even imagined till now.
Some of that tech could easily create a global imbalance in society leading the civilization to a collapse eventually.
The tech could already be around us, just evolving slowly being adopted by the global society, just as slow as most of the population needs to be it, to not collapse in a sudden explosion of changes subverting or erasing rules and status quo built through centuries. As we know from previous collapses, the rule of law and the need of societal stability to develop advanced tecnology is not easily achieved, it could take another 500-1000 years to recover from a neo-collapse ala Roman empire or Han dinasty.
Scify gives plenty of examples of revolutionary tech that if becames suddenly broadly adopted, it could tear apart most of the global societies structures.
It could be nothing like interstellar travel or wormholes, nor time travel, or else. I could be something more subtle, not inmediately possible to be perceived by homo-sapiens as a "danger Will Robinson" circumstance.
It could start simply like an - AI powered - app giving us 90% of certainty about if someone is lying in an audio sent in Whatsapp. Then you could start reviewing all recorded messages from politians since the 1930's to check if they were telling the truth or just plainly lying to their populations.
And then we could start just double-checking for lies everything said aloud everywhere.
maybe this tech needs to be conceiled by all actors because if the theories behind the weapons becomes mainstream, anyone could start making powerful weaponry just repurposing home magnetrons.
We should try to talk to them, communication. We should try to survive the first contact, specially if they are pacific.
It could not be inmediately clear that a succesful peaceful first contact could lead to disastrous consequences for us.
We should pay attention to every human instint in existence, but controlling the natural agressive response to the unknown.
But the survival instintc should not be disregarded inmediately and automatically replaced with "moderation". The aftermath of an error could be beyond a point of no return for us as species.
You have a high opinion about sapiens-sapiens capability to make reasonable actions. but most probably having surviving nuclear extintion till now was just a matter of survival instintc. Something happening almost entirely out of the neo-cortex.
I wonder how far away from Earth could have been detected the atmospherical nukes blowed in the XX century.
And if those nuke produced some currently undiscovered very far (light years), detectable energy / event? Yeah, most close neighbors could have already spotted us in 1950.
because nobody wants to say "successfully governed by a totalitarian regime"
somehow, west democracies consider anatema to admit that totalitarin regimes can effectively manage entire countries.
And China has enough success that it cannot be dismissed as luck, or anything else than (totalitarian) government plans which achieved the same good outcomes as western democracies have
From what I remember from LOTS of movies from the 80s, UFOs where always after the US military because they have stolen some advanced tech (their ship / engines to get back home), or because they have alien prisoners.
it is actually one of the few really good strategies if you want to take over an entire already populated and heavily armed planet. Of course it will take some time, a couple of decades, but at some point if you land your hefty 5km long starship in some god forgotten place, probably the shaken remaining societies won't even bother to shot some nuke at you.
If this would some kind of actual evidence point to something probably, what could it be?
Some group (of humans), have been throwing millions subtlely trying to direct the population to believe in aliens secretly visiting Earth, but only thoroughly surveying the US military?
There's something going on definitely, and why only the US military is being harassed by these guys? and the other armies are somehow being monitored, followed, by these things?
Many are discussing the alien and the secret tech points of view.
I'll talk about the "desinformation campaign", very popular in "non believers" in UFOs, nor aliens, nor secret tech.
I actually don't see the current need of keeping the 50s UFO mith alive, but it still could be a giant desinformation campaign, targetting some entities in the world, it could be a reaaaally long game of sorts, which many won't get to see how unfolds some time in the future.
It is really interesting not to have a straight answer for this.
The thing(s) will just come to us telling we overlooked something somewhere, and they were here since then, quite bit time ago.
Maybe it will be (or it was already), some innate sapiens-sapiens neurological and/or psychological blind spot, like overlooking inminent danger in exponential events.
It really doesn't matter to know exactly what consciousness is.
Because high levels of intelligence and auto-drive can fully operate complexly enough to seem "conscious".
I would start a research stating exactly what would constitute a "conscious human", maybe we could find not every human on the planet is actually conscious.
Maybe most people are just looping into some psychological / environmental / sociological context, just like scripts running happily in loop, but not stepping outside its own code, at least not intentionally.
The hypoteshis of humans being complex algorithms, beside the tabu chit-chat, implies the possibility of hacking minds just we can hack programs, looking for their bugs, triggering exploits to fully manage humans into a different setup, in order to make them work towards a different goal from what they were originally intended (having being pre-programmed by the society and/or parents).