Because they issue a lot of debt in USD. They can't print their way out of debt the way the US can and does.
Nobody wants bolivar-denominated debt. Because Caracas keep printing. The U.S. dollar has no divine right to being infinitely valuable.
The U.S. dollar is often the least silly of many options.
There's only one form of money that's retained value for over 6000 years of human civilizatoin.
With a national debt of over 30 trillion, it's amazing that some people can still believe that the US will pay all that debt back.
The USD will crash sooner rather than later.
USD bonds are more attractive since they remove the exchange rate risk, but as happened, the government could t repay the debt in USD.