One reason is that despite its precarious situation, the U.S. government is considered more reliable than many other governments around the world (which says something about the state of other countries) and is therefore able to continue to find buyers for its debt at relatively low interest rates.
Another reason is that the Federal Reserve has become a major buyer of government debt at low interest rates, allowing the debt to continue to grow and postponing the reckoning. As of January 2022, the Federal Reserve had $8.9 trillion of such securities. Where does the Federal Reserve get the money to buy government securities? It pays with its own equivalent of a no-limit credit card: printing money. Most of it is not literal dollar bills but bookkeeping transactions; the central bank can simply imagine up new dollar balances and credit them to other accounts. The result is that a lot of money is created out of thin air.
Think Right or Wrong, Not Left or Right 