You might be thinking about different "design". In this particular case USDS tries to "design" (as in engineering or architecture) efficient, cross-organizational set of guidelines to keep look consistent across all government websites. This should lead to same standard of UX and AX, which should lower amount of confusion of customers and lower costs building new projects.
You can think about it as of Twitter Bootstrap for government websites.
It depends on what you're trying to achieve. US Government sites aren't about creating an artistic experience. It's more important that digital services are delivered consistently and give citizens a good UX. As an experience, the design hinges on creating trust, accessibility, and being able to carry what you've learned about the interface to different sites. These things won't degrade easily with time. For instance, the significance of the color blue won't change rapidly (barring a large scale invasion by blue extraterrestrials).
As a designer, my job isn't to "design a website," it is to design patterns of UI/UX and a framework to build new features and interactions for a particular application. Basically, a designer's first job is to design the lego blocks. Then she can build stuff with it.
The application in this case is the U.S. Government. The purpose of this design is to provide the lego blocks and framework for developers and designers to build an interaction layer for the federal government.
No, it's fullfilled. The design of the average table, pencil, glass, fork, spoon, text-based book etc. is fairly standard (save for few ornamental details).
That's not because its purpose is lost for design conformity, but because those are peak designs that work best for what they need to do.
The only purpose design for government sites has is to be able to give information quickly and in a legible and accessible way. No artistic ambitions here. That can be standardized very well.
> average table, pencil, glass, fork, spoon, text-based book etc. is fairly standard (save for few ornamental details).
Except that's the point of the OP's comment. These items actually do not have any explicitly declared standard like this design standard published by the government. Design became standardized through people landing on a good design, not through a government mandate.
I don't think that that's his point -- I see this as orthogonal to whether there is a standard or not.
The reason those don't have a standard is merely because they are so simple they don't need a full document to describe them.
And I don't see a clash between a "goverment mandate" and "design", since in this case the government mandate is exactly a design, that is, the work and output of a set of designers working for the government.
It's not like some random bureucrat that knows nothing about the web created the standard.
You can think about it as of Twitter Bootstrap for government websites.