It's no different from anything else that's sold pre-made - shoes, bicycles, clothing. Even the cockpits of automobiles, which aren't offered in different sizes but definitely vary in size. Each make of shoes has a general "philosophy," different models will be variations on that theme, and sizes of each model will keep the same proportions. Measuring your foot gives an experienced fitter an idea of what makes and models to try.

There are still some custom cobblers around, but like custom anything, it's going to cost you.