Because America has been a superpower that has lead the world in the 20th century. Now we are in decline, but it doesn't change the role we played in the 20th century on a global stage. Whether we are the greatest country in the world is a debate for another time, but you cannot diminish our signifigance in the 20th century.
Again, until you make a valid point on why modern New World have a place in a civilization game, when there has been no precedent for this, I can't buy it. Why is this? Because if you start including these nations, then every other modern nation around the globe will warrant representation. That includes Canada, Mexico, Australia, New Zealand, South Africa, Argentina, and God knows who else. If your point is, "we are better and more significant than those countries," then you are committing the same fallacy that you are accusing me of.