ESO also isn't cross platform...

In any case, this would require every zone to be instanced so you and a friend could both enter Mor Dhona but wind up in different instances of Mor Dhona. They would have to ensure that FATEs like Behemoth/Odin especially were synced or people would be trying to pop into whichever instance the monster was spawned in. The residential zones would have like 300 Wards. It would cause chaos with the Duty Finder since even if you retain multiple lobbies for pairing you still need the network resources and computing power to cleanly route people ONTO those lobbies.