Back in the day, an oft discussed topic over Saturday night campus pizza and beer was this;
If Cro-Magnon (modern man) has been around for 50-100k years, than why has civilization only been around for the last 6-7k?
The answer seem to be in the unpredictable irregularity of the ice-ages. During H. sapiens' early days there were a series of short interglacials - most of them less than 2-3k years. Our current understanding of prehistory is that it takes at least that long for a culture to coalesce around (typically) a fertile river valley and to begin to advance into a civ. So early civs might have just been forming up in Europe and then, SMASH - the next ice age crushed them under two-mile-high glaciers.
Our current interglacial has extended well beyond 10k, so civs have formed and spread all over. Scholars have often wondered why our climate has been so fortuitous, but if anthropogenic warming is true, we may very well ourselves precipitate the next ice age.
So I suppose the point is, that if earlier cultures existed, they wouldn't have gotten very far along before new glaciation stamped them out. Early cultural remains would have been fairly primitive (no reinforced concrete military bunkers, no satellites, no plastics) and either decayed or were weather-eradicated. The few real remains, if any, may have been misdated (early archaeologists being frankly somewhat careless and sloppy) or in the hands of pot-hunters. The excavation of glacial moraines, however, reveals no evidence of these theoretical early cultures.
I'm grossly oversimplifying. By "SMASH", for instance, I mean a series of long term destructive processes initiated by radical and unrelenting climate change. These would include but would not be limited to desertification, agricultural failures, herd dieoff, social upheaval and breakdown, Internecine wars over remaining resources, zombie attacks, etc., etc.
I call this my DGSBS (Drunken Grad Student) Theory

. And by the way, where were von Daniken's helpful aliens when our ancestors really needed them?