You should read "the global minotaur" by yanis varoufakis. It explains exactly why this is the case. This is true only for the US and it is because other countries buy dollars in bulk to keep the dollar high because: 1) not keeping the dollar high will devalue their dollar reserves. 2) they need large reserves of dollars to be able to facilitate trade in dollar denominated resources like oil. 3) they buy us treasury bonds because this will raise the value of their dollars and because it ks historically a safe investment.