Cannibalism doesn't "refine" prions. It's a transmission method. Nothing more. You still have to have an initial patient zero, which is extremely rare, get eaten in order for it to spread. Kuru was only a problem because of the ritualistic eating of the dead. Once it was in the population, ritualistic cannibalism acted as a transmission method.

You're not gonna get prion disease just by eating some random dude. Your chances of that happening are more or less the same as your chances of just spontaneously getting a prion disease out of the blue without being infected by anyone or anything else (which does happen, though extremely early). Cannibalism only spread kuru because it was a widespread ritual. Once the extremely rare prions got in the population, their traditions helped it to stay and spread.