My own impression is that this is far worse in the US than in most other first world countries. If I'm right about that, then explanations based on social media algorithms etc don't really work, because the same algorithms apply in other countries too. It really then needs a US-specific explanation.

Maybe, the US has just got too big and too diverse – I am talking here about political/ideological/worldview diversity, not ethnic/racial/etc diversity – to hold itself together in the long-run. Countries don't last forever, and the US isn't going to last forever either. Of course, it isn't breaking up this year, and I think other countries are likely to break up before the US does (such as the UK, Spain, Belgium, Canada). But maybe these current events are bringing that eventuality closer to us.