They don’t even need to run a separate network. The NSA has long since figured out a way to move secure data over an insecure network. The problem is that most of the US’s infrastructure is run by “for profit” companies. And since they are neither required, not is it profitable, to have robust security, they don’t. Instead, they do the bare minimum to be compliant with whatever frameworks they are required to. And since basically every one of those compliance frameworks is all about having the right documentation and never actually audit systems directly, their actual security is shit.
If you want companies to start taking security seriously, then we need GDPR style fines when companies get breached and are found to be running operating system and software which is years out of date. Compliance frameworks also need to get into the nitty-gritty details of OS and software configuration and not just “have a baseline”.