Since when is the US the only country on the planet? It is not even Apple's largest market anymore.
I'm not making broad assumptions. Apple's whole smartphone roll-out schedule is geared towards the common 24 month financing model. And sure maybe you can upgrade early, but that costs you. You either pay more for a 12 month period or pay a penalty for upgrading early. Whatever the structure of the contract, you pay for the iPhone. Of course carriers stimulate early upgrades because it makes them money.
Still, most people do not upgrade yearly because the phone does not degrade that fast both in terms of quality and performance and simply because for many people it is just plainly too expensive. You can do a poll even on this board and would find that the majority of people here follow the 24 month model.
Don't remember saying the U.S. was the only country on the planet. My anecdotal evidence is just as valid as your speculation. Should make a poll, it would be interesting to see. BTW, it hardly costs me anything to upgrade as the sale of my year old iPhone pretty much pays for the new one.