experience. They are especially famous for being incredibly arrogant and unable to see the soccer world with even a smidge of objectivity. They are similarly thought of in Latin America, although to a lesser extent (Latin American soccer fans tend to focus their hate on Argentina and/or Mexico, and to a lesser extent Brazil, depending on the country).
I recognize that living in the US it is hard to get a sense for this, since Americans in general aren't very passionate about soccer, and since more Americans have ancestors from Germany than from any other country — which results in the US weirdly being the most pro-German soccer country on the planet, other than Germany itself. In fact, it may be the only one, other than Germany.