Crusader Kings 2's latest update (I assume you're referring to the system introduced in Conclave) butchers what was already a very historically iffy abstraction, and one that would be completely unrecognizable to medieval cultures outside of western Europe. It is an absurd fiction (and a bad game mechanic, if you'll permit me to say so) to think that kings were required to reserve places for the great magnates and acquire their explicit consent before acting. Certainly medieval kingship rested to a certain extent on the consent of the aristocracy, but there were no permanent councils as such.

The councils in CK2, with their discrete offices of chancellor, marshal, steward, chaplain, etc, are presumably based on the royal households (familia regis) of the French and Anglo-Norman kings, and particularly the great household officers. These themselves grew out of the aristocratic familia, the hangers-on that served, supported, and advised high medieval lords. The royal household was vastly enlarged in scale, and by at least the mid-12th century, many of the great officers had acquired prestige and ceremonial importance while their practical duties were largely performed by subordinates. The steward, originally, had been a sort of majordomo, responsible for the domestic functions of the court; the marshal had been tasked with overseeing the stables and horses. In England, both were hereditary by the mid-12th century, the former being passed down through the Earls of Leicester and the latter by the aptly-title Marshal family, later the Earls of Pembroke. Neither were really active, or often present, in the king's court, except at ceremonial occasions. Even such lowly-sounding titles as the butler (theoretically in charge of wine and beer), the chamberlain (originally in charge of the king's bedchamber), and the master of the wardrobe (literally the guy in charge of the king's closet) acquired importance far beyond their descriptions. By the high middle ages, virtually every officer in a royal household would be an aristocrat of some stripe. Those holding the great ceremonial offices would generally be powerful lords, but the king's day-to-day advisers and managers generally were drawn from the lower aristocracy, and thus personally dependent on the king.

I've said nothing, of course, about the importance of the household knights, an absolutely critical and tragically underrated group who functioned as the king's private bodyguard, his staff officers in event of war, his personal representatives and functionaries, and the military core of the household. They are not even modeled in CK2.

Tl;Dr: Kings would not have wanted to surround themselves with great magnates, who rebelled with some frequency; nor would those magnates have wanted to be made to play anything more than a ceremonial, absentee role. They had estates and households of their own to manage.