The old "Click to auto attack/Pseudo-Turn-Based Ticker" System is an artifact of the days before broadband. Of course they look dated.
And yet, designing an MMO requires a massive amount of planning; from zone design, to network optimization, gameplay balance, interface design (which is far more complicated than most other game genres).
Creating an MMO FPS alone would take tremendous design space and server power.
And for every variable you add to the game, the harder it is to optimize.
That's mostly from a technical point of view too.
Of course...then there's the OTHER side of the coin...World of Warcraft.
They've cornered the MMO Market in the West. Plain and simple.
It's a network good, it's scary-huge, and I too would barely call it a "game"; more like a not-game that uses an addiction model and social factors to keep people playing rather than good gameplay.
Because the gameplay in WoW can be beaten with an Excel Macro.