In the case of US, it is because the dollar is by far and away the largest international reserve currency, which gives the US what is known as the "exorbitant privilege"[0]. Putting it simply, this means few countries are going to let anything bad happen to the dollar because if it does most countries would suffer, and as a result the US can get away with things other countries could not, such as building up trillions of dollars of national debt. The same applies to the other countries whose currencies are used for foreign reserves, but on a much smaller scale given relative proportions.