>>2959641 (OP)It's because the US are a federation of sovereign states. You can see that in most of the federal states, because they are usually formed willingly by smalles countries. Like the US, Germany or Switzerland. Some countries however, grow by swallowing their neighbors, like the 3rd Reich, Roman Empire or the UK. These are empires. There are also a special kind of empire, named "Carat" and it has colonised the vast interior of Eurasia and adopted a federal structure after that.