Because there's more than just Fat Voice. There's Jack Chick's Anime Voice and Regal Voice, everyone's Hans Von Hozel impression, and Acier.
True story: went I went to the US for a business trip, I was watching TV in my hotel room and suddenly Paula Deen came on going "BUY MY HAM, Y'ALL!" That was the first time I'd really heard her and I immediately thought "oh my god, Bunnybread wasn't exaggerating."