rationalskepticism.org

There also seems to be a religious element to the issue of royalty. Leaders are assumed to be in power by consent of the favoured deity/s (how could it be otherwise?) and so if a man is so favoured by a god, how could his progeny be "ordinary"? As well as the powerful king being favoured, all that is his or comes from him is similarly favoured. Obviously, his son is more favourable to the deity that the son of some other person.

I think it's a kind of 'trial by combat' stretched out across multiple generations, with the preferred deity of the region as ultimate arbiter.