Because the word has been appropriated by folks who write about these things to mean usually anti-Jewish over the last 150 years or so but it really picked up speed during Hitler's genocide attempt.
Arguably, most Jews today are only partially Semite in reality. This is a major bone of contention and is exploited to the hilt by anti-Jewish extremists and by Zionists to further each other's warring perspectives.
Confused enough yet?
Technically....Semite ...is derived from Shem ...the eldest of Noah's progeny and if one is to believe the flood then one could assume that we are all Semite to a degree....probably. But the idiots like Christian Identity(not normal Christians..trust me) think that only white Europeans descend from Shem and that the rest...Jews in particular descend from Edom.
I'm sure I've been truly helpful here (extreme sarcasm)
Really truly...ethnic classification speaking in a technical sense...I would venture that the Phonecians, Caananites, Hebrews and all who inhabited the Biblical Palestine region are the closest to what I would call Semites.
But it's sort of like Poland....are all Poles Polish....No, a huge number of them are actually German. Same for the Czech Republic....and Eastern France....or the northern Swiss Cantons....
Ethnic classification and nomenclature is an inexact science to say the least.
regards
Another one that I like is the bastardized homo-phobia, which, if it were to be literally interpreted means "self-fearing".
Since I do not "fear myself" then it is incorrect to refer to me as a homo-phobe, even though I might maintain, for example, that homosexuality is an abhorrent, deviant behavior, and that those who practice and promote it are repugnant, reprobates .