Who says we do that? It doesn't have to be so. It is, sometimes. But generalizing the "evil" tag on all (I assume you mean dems and repubs) is much too broad a stroke to be accurate.
Sometimes Americans do get it wrong. Like the practice of slavery. Or child labor. Or denying women the right to vote. Or capital punishment. Or filibustering - that is - blocking the conversation.
Sometimes we get it right, though. Like electing our current president. Like establishing child labor laws. Like getting their heads out of their asses and recognizing that women are half the population statistically, and are - gee whiz - also humans and also citizens - duh, gee Wally, maybe they should be able to vote too - duh-huh!!!
We get it right when we elect public servants, not politicians. Sometimes it's VERY hard to tell the difference. Sometimes, sadly, the latter pretend to be the former.