They assume the underlying political system is inherently good (when the voting and legislative procedures demonstrably maximize polarization and exclude third parties) and more seriously, that both parties are good and animus felt by the public is just misalignment that can be easily corrected.

The ignore the fact of party apparatus being a giant amplification machine to promote the views of those who control the parties - a small subset of the members or even the elected legislators - via the media, and to thereby activate the larger body of party supporters. And they ignore the polarization-maximizing behavior of party leaders, who profit by maximizing disagreement. Finally, they ignore that these are not simply systemic problems. There are individual politicians who knowingly and cynically lie and and openly seek to inflict harm upon their opponents, and numerous supporters who want to be political actors who knowingly repeat those lies, marketing them to both he ignorant and the cynical.

It would be absurd to write a paper asking 'why don't human rights advocates and human rights violators get along?', yet we have this ongoing stream of papers about polarization that treat political party affiliation as a fundamentally meaningless abstraction, never engaging with the question of why adversarial partisans develop a dislike of each other in the first place.