I have been persuaded recently that when the modern Europeans discovered South America, that there were more people there than all of Europe. N/A, don't know?
If there had been millions or even 100s of thousands settled in an area the size of Western PA, they would have out of necessity build permanent settlements which would have left many more artifacts than what remain. That is what we see in Central and South America. These were hunter/gather societies who practiced agriculture on a small scale. The limitations of the land in feeding people in that type of cultures prevents their populations from growing too large.