TO be fair, the cities *do* have something the rurals need, but it's not simulated very well: protection. Protection against monsters, protection against undead, protection against foreign invaders...
The players don't really have the mindset to enforce this, though. What realm is really going to allow the enemy/monsters/undead to destroy the region because the lord wants to sell his food for 45/100 instead of 25/100? And besides, what's the consequence for the lord if the monsters do come after his region? He loses some food, loses some peasants, then the monsters wander off to go ravage someone else's region.