Probably because there aren't one million cities in the United States yet. Sure they could have done the sensible thing and numbered them in freaking order, but that wouldn't be as fun, now would it? Plymoth Rock would have the zipcode 1 in that case, right?
It seems nothing starts with two zeros, but there are a handful that start with three sixes. I want to move to Topeka now. I'm sure THEIR Nero chapter's laws allow you to raise the dead.
Schnectedy, or whatever, has the kind of zip code that idiots put on their luggage.