Because they do not enhance anything.
Either they still rely on your tone of voice anyway (most of them) or they are so outrageous that you get shock value, which enhances nothing and just draws attention to the fact that you used a shocking word, as opposed to in some sense strengthening your original point. In the former way, they provide nothing that you cannot do without them while still using tone. In the latter way, they detract from your case, rather than enhance it.
In other words, cursing is a bad habit that most of us have, which doesn't contribute anything. I still say fuck and shit because it's ingrained in me, but I also recognize that it's a bad habit.