Because the US government prints the dollar that the debt is denominated in, there is no risk that they would be unable to repay the debt, so it would have to be their explicit choice.
Technically the USA's elected congresspeople might be able to repeal or sidestep the 14th amendment and pass legislation causing the government to willingly default on their debt, but a move like that would likely cause a global financial panic and potentially even a collapse of the dollar itself, very quickly costing them many, many multiples of the amount of "money" saved. That's if the dollar is even still considered to be money at that point.
The government would start hemorrhaging money in the bond markets far before a bill like that could actually pass, due to investors pricing in the possibility of willful default. The first moment a congressperson even proposed it seriously in a tweet, the rates would be affected in a small but measurable way.
Even in a crazy hypothetical where the USA were entirely annexed by a foreign nation, it probably still wouldn't default on its debts, because the annexing country would want to preserve the asset of the USA's reputation for repayment.
So, what's the chance of the USA repealing the 14th amendment by a bipartisan 2/3 vote, then willingly choosing to surrender all of their power to other countries on the global stage? Like I said, the word "risk" starts to lose all meaning at that point. You're better off buying some books for entertainment, because you'll need to stay in your bunker for quite a while.
When you say "there's a non-zero chance that the US Government defaults", you might as well be saying "there's a non-zero chance that everyone comes to the sudden realization that money is dumb and we should stop using it".
No risk, no reward.
You've only addressed credit risk above. But come on, we all know there are all kinds of other risks at play here.
