The way I understand it, a nation's economic standing as a borrower is based basically on trust. A lender nation trusts that the borrower nation can eventually repay its debt. That trust isn't so much related to the raw number of the current debt, but rather how the lender believes the borrower will behave and evolve in the coming years. Today nobody has a reason to believe the US will do anything but continue to grow (in the long run).
If in a hypothetical alternate universe the US had a civil war tomorrow, it could have a massive budget surplus and yet it'd probably find that nobody would be willing to lend to it regardless, because the climate of repayment would be so uncertain.
In today's real world, the US has the power to print its own money. In very real terms it can define the very term "debt" as whatever it pleases. That, combined with its status as a massive buyer of foreign goods, history of entrepreneurship and growth, history of corrective action by the Fed, insanely powerful military, and so on, means that it could owe 100x times as much as it does today, and nations would probably still happily lend.
A nation is not a household. It doesn't reach some magic number "lending ceiling" where everyone else says, "you owe 100 trillion and 1 dollars, we're not gonna lend any more" like might happen to a regular person with a credit card. A "AAA" rating is just some label that some private company makes up.
So the issue isn't some dollar amount drawn out of an economist's hat, but rather the real trust inferred from a nation's geopolitical position.
Any real students of economics please do correct me, as I have no doubt my reasoning is flawed somewhere. But this is how I currently understand it.