OK, short explanation.
So, we just had Covid, a "novel" virus that NO ONE had immunity from. We all saw how that works.
It killed about 1% of the infected. Imagine the same virus killing 90%-95% of the population.
Because that's what happened to the Amerindians after the Spanish arrived. 90-95% population death rates.
EVERYWHERE.
Remember De Soto? Landed in Florida, hiked back along the coast returning to Mexico.
In the Mississippi river valley he reported "towns" of thousands. Each with a central mound that a chief lived on. He said that if you stood on one of these mounds you could see others, "as far as the eye could see".
When the French fur traders started going through the area 150 years later they found no people and heaps of bones everywhere. Everyone had died.
This is the exact same story that happened in the Amazon and across the Americas. The native populations crashed around 90%.
Now, how much "Cultural Continuity" do you think there was after a 90% population crash?
What we imagine is "native wisdom" and "timeless culture" is a hashed together pastiche of the bits and pieces that survived the Collapse. In the 60’s the Counter Culture projected their values onto Native American cultures. Most of what you think you “know” about Native American cultures is incorrect and outdated.
The populations encountered by later Colonists were the "rebound" populations that were recovering from that crash. We mistakenly thought they were "the population" that had always existed.
Here’s something I wrote about “Clovis Culture” it illustrates how our perception of Native American(s) and their cultures is inaccurate and in flux.
FYI: My Great Grandmother was Comanche. My father visited the Reservation when he was a child and this was a big issue for him his whole life. I learned stone knapping, tracking, and hunting from him.