Other nations wanted US dollars because our Treasury was reliable and we have the strongest economy in the world. They trade real resources and labor for currency we pull from our backsides at will and isn't even good outside of the US banking system. When they accumulate more than they need to purchase our exports or the energy they need they can either sit on their dollar reserves or buy our bonds. That's a no-brainer.

Our Fed is one of the main attractions for investors, as it offers free clearing services for payments denominated in dollars and our Treasury has long been the choice of investors for safe keeping of those dollars. The only thing we owe any of those is a bank statement and a small return on their savings. They prepaid the principle themselves.