Speaking as a girl with no skill in (and also no desire to spend money for) sewing, I pretty much need to buy a costume. In the past couple years, I went with the same generic black-robe-and-cape costume. It made me pretty much invisible, which was rather cool, because from temporary invisibility comes an easy scare. As I searched for a new costume this year, however, I was appalled at the amount of crotch I could see in the photos. Having tried and failed to find an already-made orange jumpsuit for my intended Chell costume, I was reverted to buying the costume with the longest skirt I could find.
This year I am a gypsy. That's...probably not the best example of sensible female costumes, but it fulfills a silly childish fantasy I had as a girl, and, again, the skirt surpasses my thighs. I also know that this isn't scary, but I don't intend to be scary on Halloween. I intend to be scared on Halloween.
...As for who's to blame for the stupid female costumes, I blame the media for sexualizing everything and I blame everyone on earth who bought into it.