Because “antisemitic” and “Semite”/“Semitic” are completely different words. “Antisemitic” is no longer semantically just the “anti-” affix plus “Semitic”. That’s just how semantic drift works unfortunately. It’s become pretty much completely disconnected from the base morpheme, and most people that use the word don’t even know that the word “Semitic” actually exists in the original sense – it’s practically a bound morpheme now, outside of its use in linguistics. Semantic narrowing is a normal part of language change.