OK, but what is your explanation?

Exactly that, South America isn't western because the idealogies of their founders mostly don't come from European individualists or eastern collectivists. They're their own category, maybe they could be grouped with Africa in the sense of tribalism, but likely each of those continents could be their own thing. Some South American countries are trying to earn their way into western culture, but I would not be the arbiter if that line.