The two party system.

They built nonsensical coalitions to get votes, so people often find that their preferred party has positions they don’t like.

Then the primary system encourages the most extreme candidates for either party.

Then you have intentional amplification of the assholes like MTG.

Then you create an artificial us versus them mentality where someone on the other side is pure evil. Obviously the GOP is far and away worse at this, but it’s across the spectrum.