Because they are future liabilities so you fund them in the future with tax revenue and/or debt.

They can keep the promise as they have been doing. The obvious point is that they would need to cut spending in other places, which they haven’t done, so money expand in currency is debased. 

They will continue to run a deficit like they have for decades. The dollar has been depreciating for over a century, nothing new here. 

 A house of cards will always eventually come crashing down.. it’s only a matter of time.