Simply because the international impact would be too big. Too many international trades rely on the US dollar. Until the Yuan takes over (speculative(!), if it would, in the next 50 years) the US is pretty much safe from defaulting.
If it really did, it'd be the end of the global keynesian economies and throw us back in pre-globalization.
Most economy would pull back from global exposure (that's non-realistic). Although we're seeing this on a small scale (when France refuses to adopt more EU policies for example, or when all EU goes pissed at Greece (particularly Germany)), it just won't happen.
What's going to happen, is massive sweeping under the carpets, "deal with it later", and a whole bunch of plans to lower debt-creep and keep face publicly.
Think of it as, bumping a massive bug to "next version" in the understanding that everyone already knows about it, and it's can of worms that would require too many structural changes, to the point that it would change the nature of the software itself.