The USD, or any other currency, is given value by the backing of the government issuing it. Any currency has a value relative to another based on the volume printed and the economics of the home country.

To have some sort of universal currency, even if someone could get everyone to agree in a name and form, you would have to sort up regional differences in wages and costs, who has authority to make it, counterfeiting enforcement, a whole host of things competing nations are not likely to agree on.