We make decisions based on partial information, processed in a probabilistic manner. The logically right way to reason involves the update of a Bayesian network - each belief ties to each other belief. But updating a Bayesian network - even approximately, is an NP complete problem.

So we not only can't make a correct decision, we can't even try to approximate it in reasonable time. We can only apply crappy heuristics. Like, "Are you with me or against me?" Hence political alignment replaces our decision making. Even for apparently pure economic decisions.