Many people today believe that mankind was visited inancient times by astronauts from other planets. There is significant evidencethat renders this a plausible theory, including monolithic structures such asthe pyramids that it is hard to believe were built with ancient techniquesalone.
However, if one accepts that ancient humans were visitedby extra-terrestrial beings, and also accepts that these beings revealedthemselves to humans and even shared knowledge and engineering techniques, thisdoes beg the question: why haven't they returned? All of the evidence for existing today is at least 2,000 years old ? why did thevisits 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 therehave been no further visits from extra-terrestrials, this is in many ways themost attractive ? it's probably the one we want to believe. This theory saysthat our ancient astronauts are currently simply observing as from a far, inthe belief that further contact could be detrimental to our development as aspecies. If this theory is correct, perhaps the plan is to reestablish contactwith us when we have achieved some technological goal, such as harnessingfusion power or developing faster-than-light travel.
2) Own civilizations have been wiped out
Given our own brushes with cataclysmic war in jus tthe last hundred years, it's not difficult to imagine that similar problems ofconflict combined with devastating weapons would also exist on other planets.It's not impossible therefore that the civilizations to which our ancientastronauts belonged have fallen victim to catastrophic war, and wipedthemselves out.
3) Time and distance
If we assume that humans have been visited byextra-terrestrials in the past, this also assumes that the beings in questionhave mastered faster-than-light travel. We know too much about the planets inour vicinity for the beings in question to have come from them, so they musthave come from much further afield. In order for this journey to be able tohave been completed in a reason time in human terms, the technology of thecraft involved must enable them to travel very quickly indeed.
However, perhaps we don't fully understand the timesand distances involved. Perhaps the beings in question mastered eternal ornear-eternal life long ago, meaning that the time elapsed since they lastvisited us is perceived as a very short time to them. We just don't know whatwe don't know here.
