Because unlike in a Ponzi scheme, the US government can simply choose to create more money, thus meeting the obligations of its payments.

A Ponzi scheme involves pretending a balance exists when there isn't one. When the gov't says "we will give you dollars later" and they give you dollars later, you might complain in the theoretical universe where the dollars aren't worth much/at all, but they can indefinitely match your bargain.

You can choose to opt out of this system! Go into the countryside, barter for everything. But perhaps, perhaps, there is a reason that economies that have fiscal flexibility flourish and economies that are pegged to some other store of value they don't control have bigger problems resolving economic issues.