Well, think of it this way: Ancient ruins are not all that decomposed, in fact, and even those that are can be reconstructed from the clues they leave behind. For example, Jericho has archaeological deposits going back 10,000 years.
We don't have recorded history of the Stone Age, but we do have archaeological evidence. We have graves, stone tools, hunting sites, pottery, and so on going back much further than 5,000 years.
What we don't have is any evidence, anywhere, of an advanced pre-stone-age civilization that suddenly collapsed into barbarism. No graves, no artifacts, no mines, no ruined refineries, no highways... not a single thing.
This is especially odd since one of the signs of an advanced civilization is a much greater population density. Advanced civilizations can feed more people, so they have more people. So somehow billions of people just disappeared without leaving even bones behind. Yet meanwhile we do have evidence of primitive societies going way, way back.
No, just doesn't make sense, aliens or not.
This isn't entirely true. There are a number of examples of what might be ancient construction on a monumental scale that is either not examined particularly closely, or has supposedly "been debunked" (but nobody seems to know when, or how, or what the structures really are, only that they "were debunked").
An example which comes readily to mind is the supposed "legacy of Atlantis" roadways found on the bottom of the Atlantic ocean. Now, I don't know that they're actually legacies of Atlantis, nor have I ever heard an estimate of their age, but I have seen two serious documentaries done on them. They stopped short of anything radical, like aliens or 75,000-year-old "lost civilizations," but they did make the observation that--while the supposed highway could be a completely natural phenomenon, a fact that they made no effort to downplay--the geometrical precision of their apparent construction is remarkable, and the paths on which these supposed highways are laid do seem quite a bit like roads. They aren't random, or meandering.
Others have suggested that there are ruins and such in Antarctica. I know little to nothing about this, even about what's been claimed regarding this, but I've never seen so much as a single, serious documentary done concerning the observation. At the very least, it should make an interesting study. The ice in Antarctica can descend for over a mile, but at one point in time--not so long ago, geologically speaking--there wasn't any. Antarctica was once tropical; this is an accepted fact of geological history. Who knows what is down there below all of that ice?
There are ruined cities in northern India that have been reliably dated to as far back as 24,000 years; that's from a National Geographic magazine now over a decade old. The writing has yet to be translated (or had yet to be, at that time) but there were things that appeared to be highways, commercial billboards, and public forums, as well as... hey. It's a city-level construction, and it's more than 20,000 years old.
I'm not one to say "for sure, there were aliens, ancient high-tech civilizations, and the like, and all of you so-called 'normal' people are nuts for not seeing it," I simply think that a lot of these so-called "fringe" theories deserve a lot more serious consideration and study than they tend to receive. The fact that a few people go nuts for the lack of it and behave completely irrationally (and un-scientifically) with regards to the evidence at hand... that, in and of itself, shouldn't serve to have the world-wide scientific community at large sweep said evidence under the carpet. Some of the greatest scientific revelations, throughout history, have come into common acceptance after years, sometimes centuries of persecution because they were wildly different from what had been accepted as dogma for all that time. Scientists have become far too inclined to go with what is *known* in recent decades, a negative side-effect of a lapse in general persecution (not that the latter is a good thing, mind). One could argue (arguably?
😉) that the sudden revelation of "hey, there were ancient civilizations 170,000 years ago that vanished 163,000 years ago" is no more Earth-shattering, relative to our current understanding of things, than "Hey, the world is round, and it orbits the Sun" was a few hundred years ago.
To my mind, ancient imagery and carvings resembling something we have now, which nobody should have had then, are significant, as are legends in ancient (10,000+ year old) Hindu (proto-Hindu? I've no idea when what is now considered Hinduism first arose) scrolls which describe the gods which supposedly brought mankind to Earth battling other invaders from 'elsewhere' in craft which are described as flying metallic discs firing beams of light and strange, exploding metal cannisters that spat fire and smoke behind them as they flew through the sky. Such could certainly be a forgery, but it's not as if it's something newly discovered; remarks about such curiosities go back further than modern rocket and missile technology does, although I'm unaware of such as far back as, say, the Chinese development of things such as firecrackers and the like.
Additionally, on a scale of hundreds of thousands of years, a great deal can change; ice ages, geological activity, acid rain (if they had advanced tech, they certainly had the consequences) and regular old erosion could wear away a great deal... who knows how long an ancient "dark age" might have lasted, and who's to say that more recent, well-known ancient civilizations weren't built on the bones of those which came before, as it were?
Things can disappear; things can go away. Dinosaurs lived, according to science, for hundreds of millions of years, an almost unfathomable amount of time. Relative to the amount of dinosaur fossils we have found from all of that time, if long periods of time were meaningless, we ought to have found literally no trace of ancient Egypt, ancient Greece, or early China at this point in time... maybe a single heiroglyph (sp), to make us wonder.