I feel like I learned everything except the question in the title. I still don't know why it's long, just a lot of facts about how long it is.
> 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.
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.