That's not why though. Nobody put that much thought into it. The reason why is that in the English-speaking world, due to the British Empire and its "American colonies," the US ended up being called "America," and so far it's stuck. By way of evidence, as early as the war of 1812 the British press was already referring to the Royal Navy's unpopular practice of boarding "American" merchantmen and pressing "American" sailors into service, a practice that ended up being an important cassus belli for the war.