You have to keep in mind that borrowing money from the Treasury but then not paying it back is also known as "printing money", and is one of the single most direct drivers of inflation that exists.
If you borrow ten billion dollars but then pay it back (with interest matching the inflation rate), then the net amount of currency in circulation is essentially the same. If you just print that money instead and never pay it back, then you have ten billion more dollars in circulation representing the same collective economy, meaning that every dollar must be worth less.
And refusing to pay back loans from private US investors would immediately tank the government's credit worthiness, making it much more expensive for the government to ever borrow money again and thus causing a substantially bigger problem.