Lords leaving the realm is for balance reasons. Otherwise lords will just be ping ponging between factions and once factions start getting defeated, the remaining factions will have like 80+ lords. Since majority of these will be fiefless, they will just run around the map raiding villages all the time.
Also when a king indicts a lord, he will lose relations with his other vassals. So eventually his most loyal vassals will then start getting indicted, then the faction will be screwed.
This doesn't affect the player kingdom as they can just reject lords.