There's an old saying. "If I owe someone £100, it's my problem. If I owe someone £1,000,000 it's their problem".
The US is massively in debt, and China holds a lot of it (although they've been de-dollarising heavily when it comes to reserve holdings). If China chose to dump all that debt onto the world markets at once, or instantly repay their debt to the US, even at a steep discount, the US economy would collapse. Yes, it would mess China up as well (and everyone else, too...) but only one of those two has an industrial base they could rebuild from. The US government appear to be so incompetent that they've forgotten this.