@OP
Because niggers are niggers and by the time anyone becomes a legal adult they've experienced enough niggers to learn this, though many people start out early thinking niggers are just different colored people, it only takes exposure to niggers to cure this confusion, and then you end up hating niggers.
Same thing happens with jews, but they're much rarer to actually encounter, and/or many you do encounter are doppelgangers who hide their jewishness and blend in with other ethnic Europeans, so that you might hate every jew you meet but never know that they're jews. At least niggr wear their difference on their skin and can't use surgery to hide the little tells.