Most explanations I've seen boil down to to "The US (or any other sovereign nation with control over their own currency) can print their way out of debt, it's literally impossible for the US to default." This ignores the devastating economic effects that printing such a huge amount of currency would cause (likely comparable to the effects of simply defaulting) and ignores the fact that sovereign nations with control over their own currency have chosen to default anyway (see Russia ~20 years ago for an example).