Let's see.
There's no physical evidence of aliens. Sure, we see alleged evidence of human-alien interaction, things like the Nazca lines acting as space runways, but we don't see any physical evidence of the aliens themselves. Unless the aliens were perfectly fastidious about cleaning up after themselves, we'd see evidence of alien corpses or artifacts left behind. To a Paleolithic society, a metal piece from an alien ship would be a relic given great esteem. Surely at some point some piece of alien debris would have fallen into the hands of men and be retained both as an artifact and in the records.
The aliens were alleged to come during prehistory. There's no record of aliens coming during the rule of King Busypants VI or during the time of any other historic person to whom we can tie the arrival of aliens. All of the arrivals allegedly occur many, many years ago. This makes verification impossible. Given how far back records go in some civilizations, like five thousand years in the case of Egypt, if aliens came at some point in the last five thousand years there’d be a more concert record than the myths we hear about. This is particularly true for claims that aliens developed the pyramids and Nazca lines as both occurred during recorded history. Equally bothersome is the fact that they seem to target prehistoric societies.
The aliens seem to have visited different areas at different points in time. Stonehenge can be dated to about four thousand years ago, the Moai to thirty-five hundred years ago, and the Nazca lines to fifteen hundred years ago. If we were to suggest that aliens had a hand in those structures then they would have had to have visited over the course of twenty-five hundred years. Were they to do so then it would seem likely that they would have visited Britain when they also came down to make the Nazca lines. At that time, Britain was ruled by Rome and there assuredly would have been a historical record of the arrivals of the aliens.
The theories are not consistent. There is a claim that myths of sky-fathers and similar gods living in the clouds are indicative of alien contact. There are also claims that the Nazca lines and Stonehenge are evidence. However, the Nazca and Celtic peoples who developed those did not worship sky-father gods instead favoring sea and Earth gods, respectively. If aliens did visit the Nazca and Celtic peoples then presumably the religions and myths of those people would have developed into sky-father worship in the same manner that the thesis supporters claim happened in Egypt and other places.
The alien astronaut hypothesis fails to recognize the skill and ingenuity of man. A major claim in support of the hypothesis is that ancient man would not have been able to construct various mega structures like the pyramids, the Easter Island heads, and the Nazca lines. That really gives short shrift to the actual capabilities of humanity. Thor Heyerdahl proved that one could cross the Pacific Ocean using a boat created from Paleolithic techniques. No one claims that the Great Wall of China was built by aliens, and that was constructed several hundred years before the Nazca lines. By claiming that man needed aliens to construct things like the Easter Island heads, the adherents of this theories fail to account for the obvious ability of humanity to accomplish great things.
Where did they all go? So if are asked to consider the past presence of aliens on Earth, where are they now? There is zero evidence for the existence of aliens at this time. We don’t see Dysonian mega structures around stars, we do not receive alien communications, and we don’t see aliens flying around now. Alien life, as far as we can tell, does not exist right now. Which suggests that it may have never existed.
Travel between the stars is a scientific improbability. Current theories as to the energy required for interstellar travel suggest it would be economically infeasible at any stage of development. More so if the trip was expected to be fast, two-way, and using a limited crew.
Alien abduction reports can be explained using physiological phenomenon. You are asleep. You wake up, or seem to, but you can’t move your body. Faces hover over you. You cannot recall where you are. There is a feeling of dread. There may be a sexual element involved. Sound like an alien abduction? Those elements describe sleep paralysis, a natural oneiric event. You don’t need aliens to feel like you’ve been abducted.
Humans take a lot of drugs. As a race we love getting intoxicated. This occasionally leads to some wild stories.
Occam’s Razor cuts against them. Most of the evidence for alien astronauts can also be explained through human endeavors. Early agricultural societies knew that the Sun was a key requirement for healthy crops so they worshipped the Sun giving rise to sky-fathers. The mega structures alleged to have been created by aliens were also well within the capabilities of humanity. There’s little need to introduce the additional element of aliens when such things can be more readily explained using other proofs.