I think MMOs are not trying to be competitive for the same reason all FPSes are so similar, and why games cost all about the same: Companies think they can make more money by simply making a clone of a successful game, instead of coming up with a new idea.
Look at games that differentiate: APB (totally failed within a few months of launching), Star Wars Galaxies (converted to a WoW clone to survive), Auto Assault(A great vehicular combat MMO that failed).
I truly wish that games would be different. But we are having so many events happening at the same time, all leading to the same thing: As games become more mainstream, and review sites become more biased as they're paid advertising funds, and big developers mimic successful business companies, it all culminates to make the next WoW or Call of Duty, which more and more people buy. Complex games are shunned as they take too much time and too much thought to play.
Now, as with most rants, my above post simply complains. The solution to all of this: make a complex MMO. Why? It's the perfect genre for complexity. The persistence that comes with MMOs, along with the ability to play either PvE or PvP, alllows for short sessions that can culminate into complex and awesome arcs. A sci-fi epic of colonizing a new planet; a fantasy RPG of continuing down into the dungeon, split up into multiple small sessions.
Hopefully games will become more complex in the next few years. Hopefully developers and their publishers will realize that complexity and breaking from the status quo is not a bad thing, and will lead to a second age of gaming epicness. That's what I hope for. That's why I shall learn to program or go into game design and help change the status quo.