There must be only one captain on a ship, otherwise disaster would surely be the result. Same thing in a communty on a little island. There can only be one man at the top. (Although in the Andes - also e.g. among the Quiche Maya and in Sparta - double command was practised; possibly a concession to the fact that there are two divine powers, one up to midsummer and one after.)

The king is like the thumb of a hand, very little would be accomplished without his force to handle situations and demands. A king is an executor of the will of god, somebody who makes order(s) and follows up.

('execute ... L. ex(s)equī follow up, carry out, pursue judicially, punish...' (English Etymology)

But a thumb without the other fingers is helpless.