Your left and right foot are almost certainly slightly different shapes and sizes, and yet all shoes come in pairs. Why can’t you buy shoes in different t sizes: perhaps a 9 1/2 G fitting for your left foot and a 10 F fitting for your right foot?
The answer is, of course, that this would make stock control for the shop a lot more complicated and at the end of the season the shop would end up with a size 5 left shoe and a size 11 right shoe that no-one, however individual, will need!
However, these constraints do not hold for internet sales or shops that use digital supply lines. There is no reason why digitally facilitated shoe logistics could not change this.
You go into the shop, find the shoe style you want, and your feet are then digitally measured. The warehouse stores shoes in a far wider range of width fittings than is possible in a shop, and with left and right shoes stored separately. In the picking line the correct pair for you is gathered and 3D printed insoles fitted to match your arch.