In the world I am building the main geopolitical power is a nation ruled by seven kings, each having their own area of authority. One of them rules over the nation's armies and there should be a reason why this king doesn't use the military to overthrow the other kings and become an absolute monarch.
The world is a classic fantasy setting, but more civilized than the usual. Magic is rare enough that it can't be used for agriculture and the like, but common enough that you would expect any important figure to have access to magic (because they can use it or because they know someone who can). Gods don't intervene in mortal business but their clergy can.
The nation in question is old and grows more and more decadent. At the beginning of the story there is no serious external threat and it has been like that for almost a century. Each throne has its own succession rules (some are elected, some inherited, some have to change hands after a set amount of time, some last until death, etc), and different things they administer (communications, food, minting, etc)
What could be a good explanation for why the king that rules over armies haven't yet overthrown the other kings? Ideally I would like to avoid magical artifacts or powerful rituals.