Because, we finance our towering debt by selling US dollars. If the US dollar is not boss, no one will be buying. Being the reserve currency means that trade worldwide is basically carried out in US dollars. If we have to buy Saudi oil and German cars with yuan we’ll be paying a lot more.
Less than 25% of our debt is held by foreign governments or foreign investors - regardless, they are still paying more for USDs than they paid 90% of the time in previous years.
The USD is the reserve currency because we buy more foreign goods than any other country in the world, we have never defaulted on our debts, and our currency trades 24-7-365 in every country in the world.
The price of the yuan is pegged to the USD, and the monetary flow of the yuan is strictly controlled by the Chinese Communist Party.
The USD might be worthless, but it is worth more than all the other currencies in the world!