Hear me out first.
PvP is simply more difficult to balance than PvE. There are more variables. Players fighting other players means there are a significant amount of interactions happening simultaneously. Passives, skills, CP, gear... it becomes a cluster. Its understandable why ZoS struggles.
PvP imbalance happens (often) when changes are made in the name of PvE damage rotations, resources, and survivability.
The solution to these imbalances in PvP is to then nerf one class compared to another. Then Jimmy the Templar gets on the forums, complains, slanders ZoS', everyone cheers, and nothing really becomes of it.
Lets just image for a second that ZoS manages to balance PvP, and it completely throws PvE out of whack. I think we all agree that's a bad thing. So what solutions do we have?
Nerf Rakhatt. Buff the Warrior. Adjust timers and mechanics. Adjust things from the E side, not the P. Sorc damage is low? Make the mobs take more lightning damage. Whatever.
Because guess what? Jimmy the Templar gets pissed, and stops giving you money. But if you nerf Rakhatt, he isn't exactly going to come on the forums and complain about it. "ZOMG I WAS KILLING SO MANY KIDS IN MY THRONE ROOM AND YOU TOOK MY BEST SKILL AWAY!"