The dollar isn’t worth what it used to be. Over the past 60 years inflation has slowly taken apart the purchasing value of the U.S. currency.
The United States isn’t alone in this respect. In 1988 Canadian economist Herb Grubel proposed a North American currency called the Amero, similar to the Euro. The currency integration would in theory phase out the sovereign currencies.
The upside is that value fluctuations could be more easily controlled. But once you have that system running, who is in charge? It would mean that a single entity would have way more power over millions.