Basically, going east<->west is nearly impossible at scale, so moving an army is not going to happen if there is any resistance.
And going north<->south is so easy, that anyone doing it could likely cover the whole country pretty quick. So odds are, if there were two countries in that area, there would be just one pretty quickly.
Definitely one of those ‘geography defines the culture and the borders’ situations.
> Chile is so long because of the Andes. Here's a map of elevation in South America.
> You can't easily pass these mountains, and the tiny sliver of land to their west is Chile.