You could argue that since the Internet has become a global commercial network, the US should no longer have these special, exclusive TLDs. But switching over would be a ton of work, and there really aren't enough downsides to the US having these domains to justify a change.

It's worth noting that .fed.us exists, although hardly any government sites us it. State/city governments often use .[state].us domains since .gov was originally restricted to the federal government, but that restriction was lifted and it's common to see .gov used for state/local governments now as well.