Unpopular popular opinion: the money we use nowadays is made up of IOUs (I owe you). The money is created by central banks. Central banks don’t get money from anything; instead, they create money. The money then goes to investors, government and corporatios, and that’s why the rich become richer, and I, as a poor person, become poorer.
Not to mention that currency from a specific country is valuable if others are interested in investing there or in their products
As less and less governments and banks want to invest in the US, they lose interest in selling products in dollars, since the US doesn’t produce much anymore, just services
So they create currency in absolute value, but in terms of real buying power they are dependent on the opinions of the rest of the world