Incidentally, any potential tellers of this joke should take great care to somehow indicate that it is merely a joke; otherwise, the target is likely to start saying something along the lines of "Because media has convinced girls that in order to be worth something as a person they have to make themselves attractive to men yadda yadda yadda", at which point saying "Because they're ugly and they smell bad!" will just fall on its face.
Worth mentioning that once I decided to tell this joke to a friend of mine, and she came up with (in my mind, at least) an equally funny answer. I asked, "Why do girls wear makeup and perfume?" She thought for a moment, and replied: "Because we like to suck cock."