Why can't the US go bankrupt?

The United States cannot involuntarily go bankrupt because it issues its own sovereign currency. The federal government creates dollars, so it can always create more dollars to pay any obligation denominated in dollars.

This was famously confirmed by Alan Greenspan in Congressional testimony when he said, regarding Social Security: 'There's nothing to prevent the federal government from creating as much money as it wants and paying it to somebody.' The US government can always meet its financial obligations - the question is only whether doing so might cause inflation.

This is fundamentally different from households, businesses, or state governments, which are currency users and can run out of money. It's also different from countries that borrow in foreign currencies or have fixed exchange rates (like Greece with the euro) - those countries can face payment problems because they don't control the currency they owe.

The US has never defaulted on its debt and never needs to, unless Congress chooses to default for political reasons (like refusing to raise the debt ceiling). Any such default would be a political choice, not an economic necessity.