Merchants Are Genetic Traitors

 The merchant caste is the only caste with the opportunity for a positive-sum fundamental interaction. Warriors define the hierarchy by beating each other up. Negative sum. Scholars define the hierarchy by beating each other up with words. Negative sum. Merchants express friendship through trade, where both sides get more than they already had. Instead, the merchant caste is the traitor caste.

 Perimeter, surface, volume. The warriors defend the perimeter, dealing with strangers who don't expect trust. Scholars are supposed to deal with the natural substrate, which can't want to cooperate, it just is. Merchants deal intratribally, meaning they're the only caste which has a real opportunity for treachery. 

 Security is affordable. This means the warriors can do their job, securing the perimeter.
 Innovation is slow. The farmers and miners and loggers do what scholars tell them to do, but the scholars can't contribute much over the course of a single mortal lifetime.
 Hunter tribes constitute an almost closed system. Result: the way to get ahead is to scam the other merchants. To first order approximation, all the positive-sum trades have already been made.

 Merchants could have constructed a stable hierarchy, letting the obvious winners win without a fight. Could have come up with cheap and reliable tests, then stuck with them. Since this is the underworld, since mortals are the children of Satan, they didn't. They went all-in on attempted usurpation. (P.S, the winners win anyway, e.g. Son Also Rises, so this produces the rat races, everyone running harder and harder to stay where they already are.) 

 Merchant instincts are all about earning trust so you can fuck over the idiot who was dumb enough to trust you. At best, it's about exploiting the moron who has no choice but to trust you, because they're so dumb trying to follow their own judgment is even worse.