Halloween costumes and girls?

Why is it at Halloween girls dress as slutish as possible? don't do anything scary as thats not 'hot' enough. I'm not attacking them, as that accounts for 99% of the females who dress up, i just dont understand the whole dressing up in an outfit from a shop instead of dressing up in something to do with Halloween... If you still wanna look slutty wouldn't a slut nurse outfit and vampire teeth be the same thing? whys horror not cool for girls?