>>2415415Because of that one episode of the Simpsons, and because there have been a couple moments in history where they decided to either surrender or concede instead of fighting. Over time that sort of thing stuck.
Like how people assume the Germans are incapable of humor, or how Americans are all fat, ignorant and lazy, or that the British only bathe once a month and rarely brush their teeth, or that the Irish are nothing but drunks.
Stereotypes exist for a reason, you know, but that doesn't mean every person a part of a group feels and acts the same. But people keep pushing the stereotypes. That's how these sorts of things continue to exist.
