Because the US has both historically demonstrated will and irrefutable capacity to pay its debt.
> It owes more than its GDP in debt repayments
So?
> If I owed more on my credit card than my yearly salary, I doubt I'd have a AAA credit rating.
If you could control the supply of currency your debt was denonminated in, you would (heck, if even without that, if much of your existing credit card debt had negative real interest rates, you might be, but you probably can’t get that for your credit card debt.)
The risk of monetization of the debt adversely impacts the value of the dollar, but not ratings of the US reliability in paying its debts, which are overwhelmingly dollar denominated.
Literally, the only significant risk to US debt payments is political fights over the debt ceiling.