A question regarding women wearing pants...

I posted this within another thread and didn't get a response, and I would honestly like to know the answer so I am posting it again here.

Why is it that the "cut" of a robe was enough to separate gender in biblical times, but the "cut" of trousers is not enough to separate gender in modern times... and if we go down the modesty path here, then neither men nor women should wear pants. Should not men and women be equally modest?