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why are there so many more sizes for women?There aren't.
Go down to a basic clothing store like The Gap and look at the different pants options available in the men's section. Skinny, Slim, Straight, Relaxed, Athletic, and Jogger aren't just different styles. Each style also fits differently at the same nominal measurements, because the nominal measurements for pants are waist + inseam but there are many more measurements that actually matter, like the ratio of your waist to thigh to calf. Pants that fit your waist might not fit your legs. Pants that fit your legs might be too tight in the waist.
If a 30 Relaxed fits you, a 30 Slim probably doesn't. They aren't just styles. They are different kinds of sizes with names.
> And while I'm it, many women I know don't wear dresses more than once or twice. Why? This seems wasteful.
Come on. Don't be a sexist caricature.
> It also seems that at least a good number of guys can wear off-the-rack for a decent fit
In my experience this is false. It depends, of course, on what you're willing to accept as "decent fit". I personally, for instance, would not accept a dress shirt that billows like a sail around the waist as a "decent fit". And yet good luck finding a rack men's dress shirt that doesn't do that if you are fit but not rail thin across the chest.