The truth is they don't. The Italian and Irish immigrants and their descendants never fully assimilated into WASP culture. You can see differences in all sorts of metrics attesting to this fact. But, and this is very important, they have assimilated better than most other non-white ethnic groups because they are closer to us as kin and in disposition. And while their descendants are not necessarily where the WASP elite is, they're phenotypically indistinguishable to the casual observer. European-Americans are still in their ethnogenesis, and this can only occur during the span of several generations.