disclaimer: i am not american
from what i've seen, they seem to be a people (or rather, multiple separate peoples) broken by defeat, and now unable to move on or develop in any meaningful way because their cultures were just way too far behind to ever catch up with the modern world. they would probably be better off today if the US had forcefully assimilated them all a hundred years ago.
like, the japanese are another example of culture shock, where they had been super isolated on their islands for hundreds of years, until one day american ships showed up with cannons and made them realize just how far they had fallen behind the rest of the world.
but with japan the gap was not insurmountable. they were backwards yes, a feudal society, but still civilized and organized, so they were able to very quickly catch up and develop into a great power themselves.
with native americans i think no such thing was ever possible, because their cultures were just way too far behind. tribal nomadic hunter-gatherer societies are just completely non-viable in the modern world.