Okay I'm an engineer and I work with manufacturing directly so this is how things work. For one thing, nothing that is produced in mass quantities is made 1 by 1. In the manufacturing world time = money. Anything that is made 1 by one is usually extremely expensive. Also although there is some human assembly when it comes to these shoes but there are some components that that are made by machine (sole/midsole). These parts are scaled differently depending on size. So different "molds" are used for different sizes. Each mold has the makes a set number parts, lets say 100 for the sake of argument. And that's how they're produced. 100 of size 7 then, switch out the mold and do 100 for size 7.5, and so on and so fourth. They're not produced by the size run they're produced by the size.
Now these grey market industries, and even replica industries, are strictly in it for the profit and to maximize it. It's Nike's job to cater to the market and produce sizes for everyone that may want one. These people have no such obligation. Why would they go through the trouble of making 100 size 15 shoes whe it known that the maket for a size 11 surpasses it tenfold? That's just simple business. The time the spent making 100 size 15 shoes that they might sell half of in a years time, they could make in a more popular and profitable size that they'd sell quickly. Production time costs money so they're not wasting there time trying to satisfy a small part of the market when they could be making tons of money on the larger majority of the market. And that's a fact. That's how manufacturing works. My company will not put anything into production unless the buyer orders a certain amount so that profit from the sale of those parts covers the cost of actual production