The simple answer is that its just artistic design.
There is nothing wrong with having women wear pants, but i believe some like to have access to dresses as well, if the items are always what they show to be, which will say pants is pants and dresses is dresses, then if you find a good item you want for your guy but it turns out to be a dress then you may not be all that pleased, as you want to wear the dress because of the stat but you dislike how your guy have to wear a dress for that.
And i be talking about dresses then, a robe is a different thing, but i have not checked how every item is in the game, is there really no pants for women? I would have imagined that more armored pants are actiully pants, armored dress is just a bit nonesensical.