It's easier to tune PvE balance because the devs can better control what's going on.
I disagree. The closest truth to that is that often developers focus on PvE over pvp, and therefore it ends up being an afterthought (Lotro is a prime example).
That's not to say that it will ever be perfectly balanced, but there's just something about the protracted rage over PvP balance that starts to get old after a while.
I think that has much to do with fighting other players. It's hard to get mad at an AI boss killing you. Players take being beaten by computers much better than they do being beaten by other players, even if the situations are equally unbalanced or challenging. It's ego.
It's the modern day incarnation of rock-paper-scissors with a twist.
Maybe bad pvp systems, but it doesn't have to be that way. Sure a lot of developers fail at pvp balance, but a lot of dvelopers fail at making games period. The vast majority of mmmorpgs are quite awful, even the pve.