This is all so hypothetical that it is almost silly, but such an explanation does make more sense to me when considering what UFO abductees are reporting.
Abductees get the strong impression that the "aliens" are harvesting human DNA to create "hybrids" possibly in an attempt to strengthen their own race.
But if Greys were truly extraterrestrial, it is inconceivable that our DNA would be of any use to them beyond curiosity. It would otherwise be completely incompatible with their own genetic building blocks (assuming life evolves independently on planets across the galaxy).
But if the aliens were actually from earth... or even from human stock it would make more sense.
Consider also that completely modern humans have been on earth for at least 250,000... modern history tells us human civilization has existed for no more than about 10,000 years, and a technologically capable civilization for only about 200 years.
There is A LOT of room for dozens of civilizations like ours to rise and fall in that timeframe... and considering the surface of the planet is in a constant state of change, is it really all that surprising that we dont find any remains older than about 10K? I wonder how much of our current world would remain after so much time, sea level changes, enviromental changes and so forth... probably nothing. Now add 50,000 to that! Maybe extremely advanced technological civilzations would even consider "cleaning up" their mess and leaving the planet to allow the earth to recover... if that was the case it would be like they never even existed.
One thing that bothered me about "Greys" is that despite their differences they look too
human (too much like a human fetus especially)
Cout it be that "grey aliens" are actually ancient humans or human relatives... maybe after thousands of years of genentic manipulation? Perhaps they've been hiding out in space only to return thousands of years after they abandoned earth?
Just a thought, anyway.