What is it about politics that makes it go that way? Is there something fundamental about humans that makes it happen?
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.