Ok, so lets follow the logic train.
The most reasonable explanation is that these crafts are Russian, German or Chinese.
They have been reported since at least WW2 by credible pilots from both sides of the conflict.
Characteristics have not changed. Since day one they have been said to have manoeuvring capabilities at forces/speeds that would kill any human pilot. They have no visible signs of propulsion and no discernible fixed or other wing. Erratic movements, none of the mandatory flight lights and more often than not completely silent. None of this has changed in the last 80 years of reports.
This suggests that it's not a new phenomenon.
But lets assume that one of the other nations have had this in their inventory for just the last twenty years.
How is it that none of those nations have ever used it to their advantage? Even using it as a threat would be a massive advantage. Why would Russia not get their craft out while the USA build nuclear silo after nuclear silo all along their western border?
Why would China not use this technology to gain the military and subsequent economic advantage that these craft would certainly provide?
Just one craft like this, could wipe out the entire US naval carrier fleet. It makes aircraft carriers entirely obsolete because a weapon delivery system that is able to deliver tactical nukes unopposed is basically an unstoppable force. Something that drops down from 80 000 thousand feet to 10 feet above sea level within a second, is literally unstoppable by any weepon or craft in the known inventory of any nation. And obviously, if this tech is from earth, then whatever country developed it would not only create one, unless it was made from some very special material of which there was an extremely limited supply. They'd likely have a fleet of these. In other words, be entirely and by far the most powerful airforce in the world.
If these "other" nations had this tech, why would they happily settle for being lower down on the food chain? I can't think of a single reason.
And further, why wait until the rest of the world catches up and risk losing the massive advantage? It makes zero sense.
It would only make sense to keep this under wraps if you were already militarily superior. For the last 100 years, the only nation that would find value in keeping this tech hidden would be the USA or one of its very close allies. Unless it's some secret corporation that is holding world governments to ransom under the threat of complete annihilation by UFO. Highly unlikely.
It's fine to assume that these might be human built and belonging to someone else, but then you need to follow that argument through and show how it fits into the real world.
These supposedly logical arguments make very little sense once you consider them for any length of time.