Oh yes, I know they have the costumes. I just wished they sold the lovely princess costumes they sell for the young girls to us who are a little older but still princesses at heart.
Well, they don't sell them at the parks- but they exist! The
Disney Store often gets adult aized costumes for Halloween and you can find a whole slew of them on ebay. I have costumes in my size for all 6 princesses as well as other Disney girls.

I've worn them to multiple MNSSHPs and P&Ps- which are the only times an adult can wear costumes in the parks.
I found this Alice costume on ebay, it's not officially licensed but looks great.
This is my Mary Poppins costume- I made this will the help of a blouse and skirt I found on ebay. I'm very proud of it!
This is a nightgown I found on ebay for only $14 that made for a perfect Wendy costume.
This Jane costume is from an online costume website.
This is my old party store variety Snow White costume- I'm 13 in this pic, but it's adult sized.
And this was from my princess birthday party a couple years ago. These costumes are all mine- Belle from e-toys.com, Ariel is the child's XL they sell at WDW (my friend in that pic is tiny, but I can actually squeeze into it too.) Jasmine is an old Disney outlet perchase, Snow White is from the Disney Store, and Cinderella is an old Disney Store from ebay.
I have an Aurora dress too but I don't have a pic of anyone wearing it.
wearing the Snow White dress at P&P
wearing the Cinderella dress while at
MNSSHP with some friends
Wearing the Jasmine costume at P&P along with a cape I found at a thrift store for $1.50
And me wearing the XL child's Ariel costume with a wig I bought and a flesh toned leotard to make it less revealing. (told you it fit!) This was just me playing around at home.
I also have a great Megara outfit that I do not have a pic of.