The first reason for this is that traditionally the writers were themselves young scholars, as were the readers (most everyone else being illiterate). Of course the male lead had no desire to be a ghoul, so that role was left to the female lead.
這第一個答案，是因為寫故事的是書生，看故事的也是書生，男主角自己不想當鬼，那鬼的角色，只好讓給女主角了。