1) A large number of countries are not democracies and are not likely to become democracies anytime soon. This makes a universally elected body a pipe dream;
2) People living in large democracies already struggle with internal conflicts between the existing jurisdictions (Scotland, Catalan, Quebec, et, al.). I can't imagine these groups would be happy to cede more power to a central authority.
3) The differentials in population and economic size would leave a lot of minority groups feeling powerless. I don't see this as a good or desirable thing.