I am tempted to say that it is because the Pukes' and their constituents--at least the more sophisticated of them--know they're actors in a political kabuki, enacted to give "legitimacy" to the self-aggrandizing, predatory practices of the oligarchy as it operates the State for fun and profit. Everybody KNOWS it's all a sham, and so nobody really takes seriously the mugging and miming and outrageousness that propels the theater. It's all a sham, and everybody knows, and the purpose is to amaze and amuse the rubes.
The Dims ARE the 'rubes.'
It is important to recognize that the relations between the two "parties" are decidedly hierarchical, with the Pukes occupying the "superior" rung, and the Dims arrayed beneath in orders of "inferiority." In this organization, USer politics replicates and reproduces the prevalent racial, economic, and social order with, in the USofA anyway.
The Dims are the "inferior" party for the very reason of their nominal existence: to represent the losers, the powerless, the social, economic, and racial "inferiors." In the economic iconography of the Owners and their designated operatives on the Right, poverty signals God's displeasure, where wealth is a sign divine approval.
The leaders of the Dims, of course, and their minions are mostly in exactly the same economic class as the leaders and functionaries of the "other side." It's as if the two "parties" were feudal rivals, with one--the inferior--clan bearing the bar sinister on the coat of arms.
But! Their leaders and leading actors are NOT permitted ever to abandon the pretense that everything hasn't pretty much all always already been decided by the time the populace hears about it. Van Jones cannot call the Pukes "assholes," even if/though by any and every relevant yardstick of such behavior they richly deserve the appellation. But Dick Cheney can tell Sen. Pat Leahy to "Go Fuck Yourself," and suffer no adverse repercussions at all...