Good point, but I find there are 3 obstacles to this.
1) Many shoes (and most jeans too) don't fit me in whatever size — either the fit is more complicated than a simple scalar, and/or sometimes I'd need a size somewhere between two of the sizes offered.
2) Returns on the scale needed are thus generally a bit of a pain.
3) I suspect that the overhead of paying for lots of returns makes these services rather dear, and since I find jeans and shoes way overpriced anyway this puts me off. (This one is probably all in my head, but it's no less real for that).
What I'd really like is a service where I could identify to it some clothes/shoes that fit me really well, and it could offer me a wide range of other clothes/shoes with a very similar fit. And isn't outrageously dear. (By all means, tell me this exists!).