There's a huge difference in population density between the two groups; there was no place in the US that wasn't inhabited even before projectile points were fully diffused across the area.

Combine that with the fact that we're temporally very close to the Indigenous occupation. Whereas there's over a million+ years of geological processes to affect those artifacts original provenance.

Also, if we haven't been looking for crudely rounded stones we aren't going to have much of a sample size to say whether or not they are, to use archeologist's favorite word, ubiquitous.

I'd wager most people aren't aware that spheres aren't common in nature.