Personally, I think the US in particular is prone to this kind of thinking because the winner-take-all system incentives a dualistic, all-in approach. Thoughtful reflection does not win elections: riling up the rank and file does, including demonizing the opponent. More parliamentary, proportional systems encourage cooperation and collaboration among opponents

As for generalizations, the English language has an unfortunate construction that allows speakers to be ambiguous about their intent when making a generalizing statement. For example, "Men are violent" is a misleadingly meaningless statement. Does it refer to some men? To all men? Is it referring to the fact that men tend to be more violent, statistically, as a group? That assertion is ambiguous, but the always-truthful qualification "some" sits uncomfortably close to the always-unfair and bigoted "all". Bigots of all stripes rely on this ambiguity.

I don't think generalizations are bad necessarily, but to honor the dignity and diversity of individuals, it's vital to avoid this ambiguity, and to always be explicit about the generalization that you're making, and then your statement can be evaluated for what it is, pro or con. E.g. Instead of a Men are violent say what is meant, clearly: Some men are violent. In general men are violent. Most men are violent. All men are violent. Now at least we have a statement that can be agreed with or disputed, instead of motte-and-baileyed