Why did American tech stay in the stone-age for so long? What stunted their growth?
They used gold, silver, and copper, and probably meteoritic iron (I forget), but apparently only for decorative items, despite the fact that there was non-stop internecine warfare throughout PreColumbian America. The copper extracted in the Upper Peninsula of Michigan prior to the (documented) European presence exceeded the known use, until it was found in recent decades that the copper in Mexico and Central America originated in Michigan, at least as often as not. Big trade networks, just like everywhere else on Earth.
Stone blades (arrows, spears, hatchets, knives) work well, and can be re-edged for continuous use for perhaps years or more. They are more effective than the metals (other than the possible meteoritic iron) when hunting or in combat, and the armor reported by Bernal Diaz (”Conquest of New Spain”, last surviving member of Cortez’ expedition) was made of cotton. IOW, stone worked, the metals they knew didn’t work as well.
If one PreColumbian culture had discovered, say, bronze, it is not yet known, and the impact of that discovery would surely have been felt by other cultures and observed by the conquistadors and whatnot.
That is a very good point and one which is hard to explain if Europeans did visit America in ancient times.
Despite that, I do think there is evidence that they were here. It might be that is is difficult to start a new civilization from scratch even knowing certain things have been done. Maybe language and some genes were the only thing they left in any amount.
If one were to dump a dozen British citizens in the middle of North America in 1000 AD they probably would not have been able to replace their tools when they wore out. I do think they would have figured out a was to use the wheel tho.