because we as humans are the only other thing we know for sure in the entire universe. these artificial divisions are kinda silly really.
it's funny that in science fiction we always idealize a unified planet "oh look captian there's flubar, the home of the flubarians"
not "oh look captain there's flubar, the home of the gits, clops, molgars(who hate the clops), bildors.....)
it's like 3 people being shipwrecked on a small rock sticking out of the ocean and drawing lines across it saying this is my part of the rock and that's yours!
in otherwords, when we are fully upright someday, the physical border of our planet will seem more reasonable than the artificial borders we've created......a representative government would work just as well for the world as it has for america.