But thats not how unstable mediveal kingdoms worked.
That may be so. But it is very much how late antiquity Empires worked, even splinters of them. How centralised or decentralised decision-making is within a Kingdom should rather be decided by the laws enacted. If you have a senate, then yes, the nobles will have a say. If you have Sacred Majesty, then perhaps less so.
However, the really big problem, come to think of it, is not the actual voting, it's more how the real aim of the lords is to provide the player with "challenge". It is utterly unbelievable that the lords would work to ruin the kingdom by starting war after war after war, even before concluding the ongoing ones. The noble houses of a kingdom should have goals to guide their decisions and their votes. With an AI so guided, the lords would not all agree all the time. They would have conflicting goals, with the Sovereign being able to support one group or another at different times. And that was very much how an unstable medieval Kingdom worked. Rarely if ever were the nobles all united against the will of the King.
The same goes for AI Kingdoms - in my current game Battania has lost half its country to the Vlandians, but they keep attacking my unified Empire, even though I do not occupy a single Battanian culture settlement. When I fight Vlandia, they do not take advantage of the opportunity to get their own back. Exactly the same with Sturgia, half of which is lost to the Khuzaits.
The AI should have a list of priorities (First, our people should be free or foreign ocupation, then we will conquer new lands or for an Imperial faction First we will reunite the Empire, the barbarians outside our borders can only be a nuisance or an ally). And they should be able to realise when an opportunity arises to acheive those goals.
EDIT: In fact, this is killing the late game, it's that bad. I'm already at war with Vlandia (very powerful), Battania and the Aserai (also very powerful). The Aserai just took Onira while I was busy fighting Vlandia - and my retarded nobles now want to attack the Khuzait, (a third powerful faction) - and there is zero support for ending any of the ongoing wars, even receiving a hefty tribute. And I'm not going to have the influence to block them this time. If the wars were all started by my enemies, I would not mind that much, it makes sense to gang up on the top dog. But this is idiocy.