Lore-wise, battles in the mists are eternal battles, like in Vanhalla(from Viking lore) Think of is as mortals vs gods rather than mortals vs mortals.
The idea is that people battle for fame and glory, it is not conquest, rather, glorifying the individual or each world in itself.
Victories, in Lore, earn the gaze and blessings of the gods. That is what everyone is going for in the Mists; it just happens that the opponents are other god-worshipping former "allies/friends" as only a select few can enjoy such benefits.
That is why lore-wise the "Mists" are a separate world and entity from the real Tyria. Think of it as a bridge between god and mortal realms, where the mortals are forever performing for the gods to enjoy, and each mortal plays a different role.