"This leaves us with but a bare handful, of which I suppose Alexander might have been one, but the odds of such a person being born into circumstances which were ripe for building an empire seem poor."
I think the answer there is that people in history (or in game) get their ability scores from a combination of genetics, environment, and development, not from dice. The resulting range of abilities makes up a 3d6-like bell curve (normal distribution), so we as gamers use dice as a prescriptive means of assuring that our in-game characters get ability scores.
But with ability scores actuallly coming from genes, environment, and personal development, we should expect to see higher INT and CHA among the well-fed, well-educated, groomed-from-birth nobility, higher STR, DEX, and CON in a well-fed warrior nobility, and so on. Moreover I'd reckon Darwinian factors were probably much more strongly at work in the ancient world.
As a side note: This conversation is making me feel really bad for Alexander the Great. I mean, what the heck does a man have to do to justify 4 18s? Conquer the known world? Become worshipped as god in his own lifetime? Go undefeated on field of battle? Establish himself as the standard by which all later emperors judged themselves? Serve as the source for the Romance which becomes the most read book until printing of the Bible? Found the greatest city of the next two hundred years? "Nah, I don't think he's all that." JEEEZ you guys are harsh.