aslanlight, the answer is simple: This planet and its sentient beings are simply too uninteresting and unintelligent for any alien to be intrigued enough to visit our world and make their presence known to us. We lack the intellectual capacity to be able to handle their manifestation, and have not yet evolved to a point where our technology can compete with anything that they already possess.
It's just as well. If they did visit us, they would probably determine that the human race is a scourge on this blessed Earth, and wipe us all out like some bacterial infection treated with a regimen of tetracycline, penicillin, and cyanide.
Besides, if they landed in Washington D.C., and they asked the nearest talking mouth hole to be taken to our leader, no one would know who that was, exactly, since we haven't had any real leadership in this country since Clinton. A debate would ensue, guns would be drawn, and the alien would get caught in the cross-fire, ending our chances of gaining the knowledge we need to survive as a species on this planet.