Probably because many words considered profane have contexts in which they are not profane. "he rode an ass," he rode a donkey vs "he rode that ass," still could be about riding a particular donkey but more likely about sex and considered profane by many. Damn and hell is viewed as profane by many who are completely ok with it used in context of religious discussion. Also what is profane is greater than just single words and it is not difficult to be profane while only using acceptable words and you can be considerably more profane than those naughty words allow.