Like Guerney, please count me out from the "we".
But if one rephrases the question to say "Why do some /so many people here on Mudcat are generalising about Americans?" then the most likely answer is : "Intellectual laziness". They cannot be arsed to consider individuals on their merits and, driven by stereotypes, they take the easy option of generalising.

Note also that I said "consider"; the truth is that some people go beyond that, and they happily judge others summarily by virtue of accent or nationality or looks or educational level or musical preferences or... the list is long. Racism is not the only "-ism" that is abhorrent.