No, you can also have an MMOFPS or just about any other game type. The reason why MMORPG's are so popular is they are relativly easy to create and adictive.
What do you mean by "relatively easy to create"? If anything, I'd say that an MMORPG is probably the hardest (and most time consuming) genre of game to create.
The man advantages of an RPG style MMO are simple gameplay mechanics, lag tolerance, low server utilization, cheap content, a built in learning curve, built in constructive player interaction, and diminishing returns on extreme gameplay. I could go into each of these to see why MMO golf / the sims / FPS / chess / poker / just about any other game type is much harder to adapt into an MMO framework. But my main point was not that it's easy to create a 3D MMO RPG just that RPG's naturaly work better in the MMO format than just about anything else.
