Longer answer: the Americas weren't peopled until perhaps 15-20kya (exact date is a matter of some contention). In the intervening time, there's a lot of inventions that happen (such as, say, the bow-and-arrow), which can have severe collateral effects, even for more distant populations. On top of that, there was a massive shift resulting from the European arrival in the New World, which starts by depopulating much of the continent, reducing the population by as much as 90%.
To give an example of how striking the difference can be: by giving guns to some Indians in New York, we turned other Indian farmers in Illinois into the archetypal Indians from westerns living in the Dakotas.
For a people to be living the same way for thousands of years requires them to pretty much be in complete isolation. Even the remaining uncontacted peoples are probably insufficiently isolated for that.
