The reason is that even if they did, it wouldn't be anything worth doing. A network with no domains and no sites other than your own and a couple of friends isn't going to do anything that an amateur radio can't do, for a lot less money and trouble.
You may as well have an electronic tin-can and string.
So while you're right in theory, it's useless in practice.
a national string of BBS mirrors?
I’m not understanding your comment. There is no reason that another internet couldn’t be made with DNS, the whole shebang.