Its really simple IMO. Its the reason why there are so many FPS on consoles. The more money a business is going to spend, especially in this industry, the more they assess the risk. The closer to the status quo, a system that has been done before, and successfully, the less the risk. What we would like is something completely out of the box, innovative, massive and brand new. To do that in the MMO genre would be more expensive than doing the same in ANNNNNNNYYYYY other gaming genre on the planet. And with it being completely different, the risk is much higher.
Highly Risky, and Extremely Expensive. These businessmen spend years in college learning to avoid combinations like that if at all possible. Again, Im agreeing with you, I would love it. But for it to happen at the scale your describing? I can really easily understand why it hasnt. Our best bet for now is the devs doing some things new, but on a smaller scale to contract a bit of the risk that comes from the game not finding a massive audience, like EvE online did.