Here is a post from reddit that puts things into perspective of how little we know about human history-

Our species' absolute earliest written records (that we've found) date back just over 5500 years. Thing is, humanity in just it's current form has been around for AT LEAST 100,000 years. That means that all of recorded history, as ancient as it may seem, is less than 1/10th of the human experience. As another user put it (comment attached below), take the total of recorded human experience, then multiply it by 20, and that's the minimum threshold of how long we've been kicking around. I'm a huge fan of conspiracies and cultural layer shit, but so many of the bizzare coincidences and similarities in global culture are actually really easy to explain with occams razor: lots of shit happened so long ago that we'll never know.

Just think, the Trojan War happened roughly 3,250 years ago right? And that was so long ago that most historians assumed the story of the Iliad was entirely a myth and were shocked to find the actual ruins of the city. Even Greek historians living less than 1000 years after the event were dubious of it's historical basis. So much of the historical narrative only exists due to one or two sources that bothered to record past events; last semester I took a class on the Hasmonean Dynasty, which existed at the same time as the incredibly well documented Roman Republic, and yet we only have one or two accounts of it on the record and those are suspected to be heavily biased. The fact is, the vast majority of the historical record can be contributed to a very small collection of individuals usually recounting first or second hand sources. Because of that our view of history is only a small, curated sliver that's tainted by all number of biases, inaccuracies and unsurities.

The most likely explanation for the vast majority of ancient aliens-style historical mysteries is that sometime in that 90,000+ year black hole of history (much longer if you want to include the cultural timespan of other hominids) there was at any given point plenty of cultural exchange between continents by peoples or civilizations that we will likely never know about. There could have been an advanced civilization that reigned for 15,000 years with a global reach and organized religion that then collapsed, fought a 1,000 year civil war that was then followed by another 3,000 year golden age (I'm just making up numbers but you get the idea) and we could still likely never know of it. My suspicion is that the commonality we see between many different ancient religions actually is a case of those religions being the diaspora of a major unified religion from sometime in the mists of the past

Honestly the absolute monolith that is deep time is in some ways more fascinating a thought than the ancient aliens concept. Like, as a historian, I'm simultaneously fascinated and deeply disturbed by how infinitesimally small our sliver of history is compared to everything we dont, and can't possibly, know. Its why native peoples with complex oral histories are so neat. Some of the Hopi people have a creation myth that actually seems to be a loose record of their ancestors traveling across the Bering Straight to the America, even recording it's disappearance into the sea. Some Aboriginal Australians have oral narratives dating back over 30,000 years that have been proven at least partially true after scientists checked their stories against tidal lines irrc.

Add on to this that any settlements or cities were likely razed as a result of time even if abandoned untouched. Many would be ground away as a result of tectonic activity, ice sheets and flooding. Most are likely buried in the ocean or in the deserts and waste of once fertile deserts. Take Doggerland for example, a massive swath of land that would have been fertile and perfect for human settlement, it was swallowed by the ocean 6,500 years ago and buried the secrets of it's culture forever. Now just imagine the costal cities (which tend to be the largest of the ancient world thanks to trade networks) that were buried by water in the ever-rising and changing costline.

If I had a time machine, I wouldn't go back and visit ancient Rome (thats saying something as that's what my field of study focuses on) but instead a random point 20 or 30 thousand years ago. Maybe I'd fly around looking for that legendary battle described in the most ancient of Hindu texts that describes flying battle machines and what appears to be a nuclear device detonating (complete with descriptions of radiation sickness). Most myths are usually based on nuggets of truth, I just would love to see how juicy those nuggets are.