[rerdavies]

There's a much simpler explanation. That those in positions of power don't believe, and those who are not do. Which makes shamans a very effective way to manipulate your followers.

Is there actually an element of randomness? Of course not. Any successful shaman knows that if you don't say what the guy in power wants you to, that you're going to wake up dead, and some other shaman is going to be doing your job.