The game boils down to this:
The battles are fun. Combat is fun. The rest of the game is fluff. Progression in the game is all about being able to participate in larger and larger battles - that's it, there's nothing else.
You start out with a weak character, weak armour, weak weapons, a weak horse, and weak troops under your command: so you an only fight against weak opponents. The most obvious and straightforward way to make your character stronger, and acquire better armour, weapons, mount and troops is fight battles, and then sell the spoils of war (loot and prisoners). Quests that auto-generate suitable-level opponents to fight against for your current level help with this progression, and are worth doing; quests that don't involve the game auto-generating a battle for you to fight are not worth doing. Arena practice fights and tournaments help a bit with this progression as well, and are worth doing (up to a point).
Once you've got to the stage where fighting bandits and/or caravans becomes too easy, you're ready to progress to being a mercenary or vassal - so you do that. By now, all quests are pretty useless and redundant. You start participating in larger-scale battles involving Nobles and elite troops, big armies and sieges - the game's progression remains logical and rewarding up to this point. But once you've done this a quite few times, there isn't anything else to progress to. There's no point in trading (independent buying/selling, with caravans nor workshops) because it doesn't get you anywhere. There's no point in owning fiefs, because it doesn't get you anywhere. There's no point in talking to notables and noble clan members anymore, because it doesn't get you anywhere. There's no point in paying any attention whatsoever to the political decisions of the realm, because that doesn't get you anywhere. There's no point in starting a family because it doesn't get you anywhere. There isn't even any point in trying to become King/Queen of a realm, or start your own, because that doesn't reward you with anything new to do either.
The only meaningful progression in the game is the progression through the scale of battles you are able to participate in, and you reach the top of that scale once you become a vassal. There is no progression beyond that, and essentially nothing else worthwhile to do in the game. There isn't any variety of pathways to this progression, either - everything except fighting battles is just a side-show, because it doesn't get you anywhere. Fighting battles is fun; the combat is fun. But after you've fought battles over and over again at the highest level and scale possible in the game, even they begin to get boring. There is nothing else to look forward to or work towards, nothing else to achieve. There is nothing else to do... so you stop playing.