This is a question I explored using the Google some years ago during the many hours I played like a rock in AC waiting for decent starting cards. And it seems to be because of a combination of our voting systems (almost universally plurality voting) with single member districts. And there is even a political science name for this effect: Durvinger's Law.
https://www.minnpost.com/eric-black-...o-party-system
P.S. Because we have an early 19th-century election-and-representation system.
P.P.S Enjoy learning how we are fucking stuck in that sub-optimal election/representation scheme!