Besides the fact that being a Rabbi today is not sererah in most cases unless he is a dayan….Like I said, because its an optics, societal and political practice that most nations and peoples have historically enforced. It was the tzurah of klal yisroel. The Torah often takes ancient practices and elevates them. Converts and immigrants to other nations historically have been oppressed and abused. Thats why the Torah says we must not oppress converts. Women of other nations historically have been raped during war by the opposing army, thats why we have the mitzvah of yefas toar- to discourage rape.
Similar to why in America one cannot run for president if he is a “ger”. He has to be born here or his mother must be an American citizen if born abroad. The president of the united states is the ultimate position of “serarah”.