There is significant evidence that renders this a plausible theory, including monolithic structures such as the pyramids that it is hard to believe were built with ancient techniques alone.
However, if one accepts that ancient humans were visited by extra-terrestrial beings, and also accepts that these beings revealed themselves to humans and even shared knowledge and engineering techniques, this does beg the question: why haven’t they returned?
All of the evidence for ancient astronauts existing today is at least 2,000 years old – why did the visits stop.
There are a few theories as to why this is the case:
1) Desire not to interfere further
Of all the theories that are possible as to why there have been no further visits from extra-terrestrials, this is in many ways the most attractive – it’s probably the one we want to believe.
This theory says that our ancient astronauts are currently simply observing as from a far, in the belief that further contact could be detrimental to our development as a species.
If this theory is correct, perhaps the plan is to reestablish contact with us when we have achieved some technological goal, such as harnessing fusion power or developing faster-than-light travel.
2) Own civilizations have been wiped out
Given our own brushes with cataclysmic war in jus t the last hundred years, it’s not difficult to imagine that similar problems of conflict combined with devastating weapons would also exist on other planets.
It’s not impossible therefore that the civilizations to which our ancient astronauts belonged have fallen victim to catastrophic war, and wiped themselves out.
3) Time and distance
If we assume that humans have been visited by extra-terrestrials in the past, this also assumes that the beings in question have mastered faster-than-light travel.
We know too much about the planets in our vicinity for the beings in question to have come from them, so they must have come from much further afield.
In order for this journey to be able to have been completed in a reason time in human terms, the technology of the craft involved must enable them to travel very quickly indeed.
However, perhaps we don’t fully understand the times and distances involved. Perhaps the beings in question mastered eternal or near-eternal life long ago, meaning that the time elapsed since they last visited us is perceived as a very short time to them. We just don’t know what we don’t know here.