there is a reason it is called the United States of America even though all nations in the Americas are part of America. It is a claim to rule the whole of the Americas.
Of course that is not the whole world but we are not finished on this side of the globe yet.
You may be joking, I'm not sure, but the United States of America was named that because it was a collection of united states...in America. Expansion into Spanish territory wasn't even slightly on the radar at that point.