Going to drop the normal "well you can do it in Warband" - which I guess is right enough, but I do agree, you should be able to do so. There is not a RTR concept in Bannerlord but even from a historical perspective it wouldn't be uncommon for generals or other high ranking members of an "inner council" to be elevated to nobility (or Emperors, during the Crisis of the Third Century)
I'll digress on the historical precedence, but it would make for more enjoyable gameplay, I usually go for 300 Trading to grab a town and a castle and when I pop a Kingdom I almost immediately get declared on and just having companions in Parties does not help too much. Making them Lords would help with fief management as I expand and I believe their party cap goes up a lot higher (and you don't need to pay for their parties, either).
I would also extend this idea that the AI should also hire wanderers, it would be fun to get into a battle and have to fight and lord and their "champions" and it could help the AI maintain the number of Clans / Lords in their own Kingdoms towards late game, especially when Lord death in Auto-calc is added. Right now, without minor factions marrying, and with children not coming of age before a clan is wiped out (due to a lack of an "orphan/adoption" mechanic), late game ends up being a very dull affair of chasing down the same party of 50 recruits until I can land a killing blow on a Lord or get annoyed and execute them.