I figured it's more of a backwater on what is mostly an impassable zone anyhow. There would likely have been some river civilizations that had a lock on trade through the area, but there is still plenty of room around the edges and a few minimally-obstructed rivers to allow for silk road sorts of things. It's the only area that's two other areas from the sea (except through Diamond, but that's a long and narrow area that's still 2600 miles to that coast. Big continents get you big uglies. It helps if they don't much like water, too. Perhaps the underlying reality is a trifle different than what those around the edges actually experience.