Authoritarian forms of government will be unstable when there's no clear line of succession. It's the reason why monarchies come about. It's not like people don't understand that it's extremely arbitrary, it's just that it's better than the alternative: civil wars whenever whoever is in power dies. It's an agreement among various lords that it's better they just accept that person over there that's the son of the King will someday be King rather than having a civil war when the King dies.

And sure civil wars would still happen, but most of the time the succession would happen without bloodshed. When there's no line of succession, it's just constant power struggles when the leader dies.

Of course democracy is a far better way to determine who will run things, but that requires a literate population to work. Which didn't exist throughout much of human history. Sure, there were republics throughout history, but they'd usually become monarchies when the illiterate masses would decide they liked that Julius guy (he threw the best parties!) and hated that Brutus guy who killed him, even if the Julius guy was becoming a tyrant.

So monarchies suck, but they're better than civil wars. So when the population becomes too dumb, it sucks, but it's better than the alternatives.