The US government is ~37 trillion dollars in debt and rising. They're not going to do anything to cut spending until they hit a crisis so massive that they just can't spend any more. Which is going to have to be a shocker of a crisis given how they've handled the last couple. Raising taxes seems to be off the table too.
While it is true that US treasuries are low risk, they are low risk in the way that lending the broke town drunk money are low risk - you can come up with accurate estimates of how likely you are to get the money back. The real mystery has for some time been why people persist in feeding the US government money voluntarily. The math doesn't seem to be ambiguous, the value lent can't be paid back in real terms. Foreign holders of these things are engaging in charity.