The developers think that having a chance for a good item is more engaging than getting a guaranteed predictable item after working for some hours.
They want slot machines.
The only real results are more addicts, and not more engaged players. As that skinner box effect is highly addictive.
Yet, it gives no real fun gameplay, and people burn out doing things just for the rewards.
Actually, the WoW devs are modern drug dealers, without selling physical drugs. They sell mind drugs.