Well it is your opinion, and not necessarily applies to every player. You grind BGs and arenas 1st of all for gear, same like PvE, after that there is like PvP endgame when you are facing equal geared people and this is he part where skill matters the most. PvP battle are never the same, because you are fighting different people, different comps, they may execute different strategy, endgame PvP is all about adapting to situations, countering, forcing CDs and timing, there may be so many situations where smth doenst go the way you want, and it is real challenge. Competitive PvP also requires teamwork and coordination, same as PvE, but the main reason why PvE is like smashing a dummy compared to PvP, is because after completing a content dozens of tims with you core group, where everyone is doing their job, there is nothing left. You execute same strategy all over again (you might try different strategies, but it is absolutely not necessary), while in PvP you must adapt to your opponents (everyone will try to take advantage of every error you made), each class have many spell to be aware off in so many situations, arenas combine more classes together, which makes it even more wider, and in BGs where also objectives come in play. PvE boss have way less spells compared to all classes together, and their mechanics are going just in loop every time you start that boss again, and again.
If you think that PvP doesnt require teamwork and coordination, then sorry dude, I think you never PvPed with/against high rated players, maybe be you just dont see this as competition.
About guild cooperation - how exactly it is different from PvP guild? You are just joining forces to raid together, the same as ppl from different PvP guild join together to make premades and counter opposite faction premades.
PvP offers season, that always give you different rewards.
After your core group got BiS gear, there is no diversity left, the bosses will remain the same, as their old mechanics. The only thing that changes in PvE is ppl you are raiding with, at first it might be a challenge to teach everyone to do their part, but after they learned the strategy, there is nothing left, you just faceroll the bosses again, and again.
This is my point of view, I expect that not everyone agree with this, but I think majority of PvP dedicated players think approximately the same way.
I expect that PvE dedicated players will defend their part of game and say PvE is never the same...you just cant face reality, PvE is the reason why Blizzard have to make expansions, so there could be smth new to do, while PvP can stay the same and competitive spirit will not cripple because of that.