Although they have already been nearly insolvent a few times during the last years, the USA get negative real interest rates on government debt.
That means the market believes that there are no alternatives with sufficiently low risk.
Hopefully the market is not wrong.
The US credability will be lost soon and the US dollar will crash.
Buying bitcoins using fiats has nothing to do with this by any means. (If dollar crashed out, we just need more dollars to buy one bitcoin.)
It is possible that the US dollar price will be much higher in the near future.