I think that during those times, they were made identical in pattern and shape was because it was easier, and less time consuming. I think that having different patterns would need alot more tailoring, and more time. I think that it was also used as something very simple, a piece of clothing used to keep you cool in the summer.
But now after reading someone's else comment, I do agree with the fact that women do not go outdoors alot and it is very possible that when they do, they do borrow the clothes from the males in their familes.