I think you are mistaken. An Internet is formed by connections between many computers. But if none of THEIR computers are connected to any of OUR computers, they have their own Internet. Every connection has to go over some kind of wire or radio link. Those are mostly controlled by the state, apart from a few satellite uplinks. Then, they can make sure that the Iranian Internet addressing system conflicts with the "real" one, for example by reusing popular IP addresses for essential services in Iran. Same for the DNS servers. They would only resolve addresses within Iran. Heck, they could build their entire Iranian Internet on IPv6! Wouldn't that be fun! And if they change the protocols just slightly, it would make it very hard to interwork/tunnel with the rest of the world. There is nothing new in any of this. But it's hard to do, because you would have to develop a whole lot of stuff from soup through to nuts to make it work, and nobody else will have any incentive to help you with it.

I think there are a number of issues around this.

Firstly, graphics hardware makers prefer to keep parts of the driver proprietary, so they are not tied to published interfaces. This reduces the amount of documentation they have to develop, and allows them to make minor changes in the HW/SW interface when they need to without impacting anyone else.

Also, this is an area where Microsoft (and to a lesser extent Apple) have huge experience. Both companies put a lot of resources into making their platforms easy to develop for, and that includes a lot of help for people writing drivers. Microsoft's WHQL (or whatever they call it these days) runs training sessions and labs with Microsoft experts attending, so the hardware folks can get their questions answered by people who really know the code, and have helped many other people do the same thing in the past. This is nitty-gritty stuff, and costs a lot of money to staff, prepare, and keep going.

Of course the motivation behind this is to make it easier to code for Windows (or MacOS) than for other O/Ses, thus keeping the quality higher and improving the consumer experience. The hardware folks are only too happy to take advantage of this, so vendors write drivers for Windows first (both for the size of market and because all this assistance is available). And so the cycle continues.

I think it's unfair to say

It would be more fair to say "Until the Linux community provide equivalent tools and assistance". And for that, someone has to see money in it. Don't expect the hardware guys to pay extra for the priviledge of adding Linux support; they get paid the same for their hardware no matter what the O/S chosen is.

...where they mean when they say stick this where the sun don't shine! :-)