nothing is stopping them, other than the fact that it would accelerate their financial crisis. It would both increase inflation and drive global bitcoin adoption as a treasury reserve asset. The US wants the treasury reserve asset to be US treasuries, not bitcoin.
If the USD itself is not pegged to bitcoin then the destruction of the US bond market will continue. You cannot, as you said in your previous post, simply "borrow against your bitcoin". The US is spending 3 trillion a year. There is no way for them to somehow buy trillions of dollars worth of bitcoin and then simultaneously borrow against it in the amount of 10's of trillions of dollars.