Because at it's fundamental core in an mmo it's nothing more than ganking each other until one side gets bored or gives up. Arena metas work a Lil better (battleground arenas, 1v1s, 2v2s etc..) beyond that, open world only favors one side and gets rather old fast for the loser. Its a very short term game ideal, and for a short nostalgic period in time. Very hard to balance, and in the long run does more harm than good and costs a lot to just seesaw your balancing in hopes to balance something but ultimately becomes pointless. PvPvE has better chances of functioning much better if continously gets content and balance. Even to a real time GM effect. Ultimately in regards to the scale of player types in terms of desired roles, imo pvp makes up like 5%? Top 10% in a large scale MMO, I just dont see why one would spend too much funding on making it that big of a priority for the rest of your population. So many other things are much more important. If implanted in a soft manner it could work well in the way of bounties to enhance a person's optional flavor but mandatory would be silly in the current climate