Modern South American Population:
Pre-Columbian Civilizations:
Part of the reason for that is that the places where there were fewer natives ended up having fewer colonists too and consequently lower populations in general. So yes, they had fewer natives, but there were also fewer Europeans to dilute the native influence. The introduction of African slaves was obviously hugely influential for the demographics of Brazil and the Caribbean.
Certainly though there was much more reluctance to interbreed with Native Americans and Africans in the United States.
Still, if you see my last posts, I didn't even start to talk about Brazil or the Caribbean, where the Africans was dominant. In a phrase I was specific about countries like Colombia and Venezula, where the African influence is limited to the coast that isn't as populated as the Cordillera; and Chile that, even though it was a part of the Inca Empire, it was a peripherical part much less populated than today's Peru and Bolivia. All three are Mestizo-majority countries.