Broadly speaking, the incentive the US government has to repay its debt is the ability to borrow more in future. Also, the ability to borrow at very low interest rates.
Investors are prepared to lend money to the US government at low rates because it is a safe bet: the US does not default on its debts. I don’t believe the US has ever defaulted.
Creditors assume that the US is a good investment because it has, up until now, actually been a good investment. Low returns, yes, but very safe. That would change if the US defaulted.