You're assuming the "spheres" in the cosmology chart (Void, Light, Death, Spirit, Order & Chaos) are physical locations, but I don't think they are: There aren't "Deathlands, Lifelands, Voidlands, etc." They only represent spheres of influence. Sargeras wasn't "hiding in the Chaos realm" he was just far away in space. That's specifically why all these forces are fighting over the Dream, The Planets & the Shadowlands: It is these material planes that actually have any real estate values. No one has territory that innately belongs to them, not even the cosmic forces.