> What is it about politics that makes it go that way

It's not “politics”, it's the US electoral system, and particular:

(1) single-winner majority or plurality legislative (and state executive) elections and

(2) Indirect federal executive elections where the voters are elected in winner-take-all plurality elections at either state or a mix of state and district levels.

Together, this strongly encouraged a system of two major national factions, each of which strives to acheive a winning coalition with as little compromise on policy priorities as possible, which in effect means minimal size to acheive victory.

Yes. Read a conflict of visions by Thomas Sowell. It explains why people separate into these two camps across time, place and culture.