China doesn’t let the RMB float freely, so trade still happens in dollars. Also, the American government buys a lot of treasuries, so it’s easy to save a few billion quickly when you need to, which isn’t really supported by any other currency…saving money by lending it to the USG conversely prevents it from re-entering your own economy and stoking inflation (China isn’t the only country to use the USA like that, Japan buys more treasuries than China usually).