The NSA does perform that function for the government. They protect DoD and IC assets and critical civilian computing infrastructure. They created SELinux and s

The NSA does perform that function for the government. They protect DoD and IC assets and critical civilian computing infrastructure. They created SELinux and sponsored many of the major cryptographic standards out there. They don't actively provide defense for iOS and Android because those are product owned by trillion dollar private companies who can pay for their own security, not expected publicly-funded agencies to do it for them. The Internet is an interesting case. Nobody owns it. It isn't even American. The fact that it was originally created by and for universities that all implicitly trusted each other has led to a whole lot of security flaws baked into the core assumptions of the most basic protocols. But the NSA does protect the hell out of military networks. Military and IC networks are absolutely nothing like the Internet. There is an inherent difficulty in bringing the same assurance to public networks, though, because nobody on a military network expects to be anonymous or to have any privacy. Users implicitly trust the network's central authority. They have to because they work for it. Security is a lot easier with a trusted central authority.