They could default on their creditors (other governments like China who loaned money to the US government). But this would spark a global financial and banking crisis.
They could default on the Federal Reserve, which owns trillions of dollars of US debt. But this would create an epic currency crisis for the US dollar.
They could also default on their obligations to their citizens—primarily to future beneficiaries of Social Security (who collectively own trillions of dollars of US debt).
Or they could choose to default on their obligations to every human being alive who holds US dollars… and engineer rampant inflation.
None of these is a good option. And simply put, the US government has reached a point of no return.