So, in the late part of the 22nd century all the countries and people of earth combined to form the United Terran Federation. Its capital is in DC, but each former country has its own capital in its former capital city. The UTF is overseen by a High Councilor and his administrators, and they make a decision about what to do with Earth. They combine each region into one city, e.g. East Asian Megalopolis, West Coast Megalopolis, South American Megalopolis. My question is, what is a plausible explanation for why they do this?