I completely agree with you, but I also think you cannot hand wave away the small differences. Islam is such a perfect example of this. Sunni and Shia Muslims have been literally killing each other for about 1400 years now. Why? One side believes that Mohammed's father-in-law should have been his successor, the other side believes that his son-in-law should have been his successor. Their fundamental views and values are identical. There are some derivative differences in the value they give to various supplementary texts to the Koran, but all in all - they are still pretty much the same. Certainly near identical compared to any outsider. Yet that difference is enough to justify thousands of years of murder.
People don't tolerate differences well, even when they think they do. For instance how would you like a representative population of the US in your neighborhood and work? People tend to think about things like this in terms of colors instead of views and values. That'd mean, among other things, that of every person who voted in your work/neighbor, more than 46% would have voted for Trump.
Ultimately I think politicians are reflections of the people rather than their directors. Did Abraham Lincoln end slavery, or had the public mind changed in such a way that the election of a figurative Lincoln was inevitable, even if by a different name? So too in modern times. It's not like America was just cheery before 2016. Ever since the normalization of the internet we've been becoming extremely divided. And so it's only natural that our politicians would in turn end up being ever more divisive.
Think about the migration debate. It's not about things like trying to determine what a sustainable level of migration is, or its longterm economic and social impacts. Instead one side wants to build a wall and block migration as much as possible while the other side is dog whistling for open borders and literally suggesting free medical care and other benefits for people who enter illegally. Both sides are basically just trolling each other, and I think that's a reflection of what we the people are doing to one another. And of course this is not just intra-America. See, for instance, the huge rise of 白左 / baizuo. People view things through difference lenses, even when we do have nearly everything in common.
Maybe that is even actually the real point. The more you have in common, the less of a difference you need to create a meaningful divide. When you disagree on many things, one more disagreement isn't really changing your relationship much. When you agree on nearly everything, one irreconcilable disagreement is suddenly a big deal.