Because you're still seeing dozens of players running through your phase, even if they can't see what's going on in your version of the world. It kind of ruins the event, makes it feel less significant and less dramatic.
Yeah, and also the fact that I know the game would fall apart if the landscape changed too significantly - as it would make questing and socialising with others impractical.
Let's say you burned down a city or castle - and you quested in that same area with others. They might have chosen something different, and they might see the full castle there while you see only ruins. Also, if you eliminated enemies and they didn't - they might start fighting invisible enemies right in front of you, or things of that nature.
It's technically feasible - but I think it would be too jarring when playing in a party.
They still haven't figured out a good way to solve this problem.
Personally, I think the way to evolve the genre is to actually ALLOW massive changes to the landscape, but also to implement a sufficiently sophisticated element of simulation and a dynamic AI - where NPCs/monsters would be able to rebuild or repopulate cities and other areas.
They should really look into how to re-use linear story elements in somewhat randomised quests, so that you'll never go on exactly the same quest for the same person as other players.
An obvious tool here would be player-created quests, which is extremely rare in modern MMOs.
But, as of yet, developers have to limit themselves a LOT in terms of what's possible here.
But the world does change a lot more than what I'm used to in an MMO, like the time I had to stop a ritual which caused the whole sky to darken and a hurricane to rage.
Yeah, in terms of small contained stories and temporary changes - it does a lot more than most MMOs.
That said, WoW has also done some very significant stuff with phasing technology. So, it's not entirely unique in this way.