* The media wants sensationalism, because sensationalism gets them peoples' attentions and thus money. There is nothing more sensational than riling up and pleading to peoples' emotions.
* The political powers that be want to keep the people divided. A divided people is easier to control and manipulate, whether it's smear campaigns during elections or undermining the safety mechanisms of our governmental institutions or straight up cancel culture.
How do we solve all this? Beats me, I'm in my mid 30s now and already tired of all this bullshit. I'll spend the rest of my days by myself in the countryside just enjoying the simplicities of life while y'all go and club each other silly.
The American people have always been divided. Until semi-recently (~1960s) the political parties didn't sort themselves neatly on these divisions but over the last 30-40 years they have.
To be tribal/clannish/insider-outsider also seems to be an innate human trait.
Ezra Klein wrote a good book on both the (US) political history and various psychology studies on it: