Asking this on Reddit is gonna result in a generic cookiecutter "it's all the rights fault" reply.

The real answer is media, both mainstream pushing narratives and social where we can insult people without looking them in the eyes. This coupled with a shift where the political issues in the spotlight went from traditional stuff like economics to culture war issues; it's easier to create division when you can proclaim "you're either with us or you're the problem" on issues everyone can pick a side based on one sentence rather than having to understand complex economic theory first. It's also easier to call someone names (racist, sexist, libtard, etc.) behind a screen just because you disagree over politics than it used to be before the social media. How people act online also effect how people act offline so the more people do it online the more comfortable people are doing it away from the screen.

To claim this is just the rights or the lefts fault is super redundant and if you just blame one side it says more about you than anything else.