Why is the US government even supporting specific browsers? It should support a standard like HTML 4 with no CSS or JS required. i.e. make the actual functionality on their sites simple to reduce the chance it doesn't work somewhere. They're not Spotify. They're not trying to growth hack. They don't need to look pretty and have fancy animations and match some designer's dream down to the pixel. They can add CSS etc.…
You're right of course.
It's not really a case of 'supporting' browsers, it's a case of testing their sites against other browsers in case developers have accidentally written some non-portable Chrome only code.
This was very much the case in the IE6 era. Developers wrote and tested their sites for and with IE6, and were then surprised they rendered (in)correctly on Firefox and looked wrong. At least these days there are shim libraries, rather than having to explicitly rely on things like the box-model hack.