Answer:
Earth is not flat because the spheroidical shape provides a more effective shape in terms of exposed surface area for absorbing and radiating radiation, and also for a relatively more uniform gravity pull on different points on its surface which might not be so for a flat surface.
You live in your country, not mine because you have evolved over time to be more adapted to living in your own kind of envoirment. Members of a certain species (like you and me as humans) have been spread to maximize the survivability of their species compared to if they were clustered to a particular place.