The contemporary Merovigian and Carolingian rulers were not absolute kings either. They need to convince their vassals at the annual marchfelds to join a campaign or ratify a policy and if they failed to build up consensus their plans would go down the drainer. Nonetheless, they did have wealth and large retinues and influence that put the a league above everybody else and the ablest of them could use the promise of rewarding their followers with office, land or loot to sway the opinion of the vassals much more often than not. Horik has none of these. He is no king at all.