Argentina is a serial defaulter yet investors still lend money to the government there.

If the U.S. decided to completely default on it's debt it would be more destructive to the U.S. economy since the majority of the debt is held by U.S. citizens & institutions. Insurance companies & banks would collapse instantly since the a very large percent of their asset bases would be wiped out.

We (meaning U.S. citizens) owe more to ourselves than to any foreign entity. Default is, at least right now, a political non-starter.

"Hey, POTUS, you got us. BTW, we'll never lend you money again. Or vote for your party again."

The US would never need to default on its debt, since its debt is in USD. Worst case, it would just print more money, leading to inflation.