Because white people mog.
At least, that's always been the case.
How does a shitskin, to whom we brought civilization, want to insult us?
However, there are numerous relevant slurs between different white nationalities. Such as "spaghettis/spaghetti eaters"(Spaghettifresser) for Italians, "frog eaters" (Froschfresser) for French, or "island monkeys" (Inselaffen) for English people (from a German perspective).
"Polack" for Poles, "Käsköpp(e) " (cheese heads) for Dutch etc.
