Wow Heather - you have written this with palpable zest! Electric, succinct, direct writing. Thanks so much, a pleasure to read. Bookmarking this for future referral as there is so much to take in. Amazing research and examples.
My take on why we think like we do is based on the takeaways from Jonathan Haidt's excellent book - The Righteous Mind. It goes someway in explaining why we are so tribalistic. We are essentially intuition-driven information processors, constantly scanning the world around us, morally interpreting information and colouring narratives with our unique worldview. We readily allow intuition to lead us and then use our rationality to come up with evidence to support our beliefs - the absolute arse-backward way it should be.
I'd really appreciate if you could read my article and feedback - the parallels between your take for WHY we are so polarized and partisan and HOW our brains are literally wired to be this way according to Haidt, are uncanny.