Because, if we were truly that smart, we would already know, there would not be multiple debates on the topic, and there truly would be one God. Human beings, much like the animal world, are very much like animals. Mostly, because we are animals. Our behaviors and instincts very much model all other animals on the planet. International boundaries and a world without boundaries amongst human beings will never exist unless our world is attacked by an alien force, which, is possible.
Snakes create their own boundaries in their dens, bears, wolf packs, wild dogs, fish, etc.
Alot of wild animals define boundaries and defend them until death. The reason they do so is probably a natural instinct to do so. Man is not so different from the animals. Guess what bud, how does Joe Blow scientist truly know that the deer do not telepathically connect to one another to signal danger. In all reality, he really doesn't.