The other possibility, is that most of their tools were composed of biodegradable stuff, like wood, bones, and shells.
In Africa, they used wood and raw iron for their stuff (I have spears, shields, and arrows, from there). That doesn't tend to age well.
If you walk the mountains of Nevada along the fossil shoreline where the Lahontan[0] civilization thrived, you will regularly find large quantities of neolithic trash strewn about. Broken, half-finished, or abandoned stone tools are ubiquitous. It takes little effort to find obsidian arrowheads, some in perfect condition.
The preponderance of the evidence is that these were viewed as disposable artifacts, use once and throw away. If they were actually valuable, they could have picked these up in the same way we pick them up today. It would have essentially been "free money" lying on the ground. Yet for some reason, in all the intervening millennia, so few people did that modern humans can find them everywhere.
Those tools were really valuable. They busted their butts, making them, and wouldn't want to toss them aside, just because they got chipped or cracked. They are much more likely to throw them into a "spare parts bag," and keep them around.
Today's throwaway culture is actually pretty new, in human history. Just hang around some poor folks, to see radical conservation, in action.