I have always attributed much of that to our two party system. It encourages people to think in binary, and that binary is becoming more Manichaean by the day.
I think we're close to the same page.
Old style partisanship, i.e. Democrats vs Republicans was not a problem. The Republicans promoted individual initiative, entrepreneurship, encouraged the cream to rise to the top which gave them the reputation of being for big business but it wasn't that at all. It was simply the recognition that the more business prospered, so would the people. There is no reason to even have businesses if it is not to sell products and service to people who can afford to buy them.
At least from FDR on, the Democrats were more into homogenous society with government driving the process with more and more laws, regulations, mandates. There were exceptions. Nixon stuck us with the EPA for instance.
But all in all the America I grew up in was pretty unified in its values, customs, traditions, sense of right and wrong. Democrats and Republicans might heatedly argue process and policy, but in the end would shake hands and go have coffee together, attended Lions Club or Elks or VFW together, attended high school concerts and sporting events and Little League together and were neighbors helping neighbors.
I think there were several factors that began polarization of Americans sometime during the 1960's and 1970's and though it was barely perceptible then it has steadily increased over the decades until left and right barely speak to each other, much less are willing to work together.
It might have started with the JFK assassination, certainly the Hippie Movement and its resulting factions and Vietnam were large factors. LBJ's Great Society initiatives created some new militancy that we hadn't seen much of before.
There wasn't any one specific thing we could put our finger on but I think there was a deep state forming through all that. And it drives most of it now.