I think you don't understand what Massively multiplayer online means. Yes, COD is one.
From the undergenre of FPS.
For example, Singularity is a FPS. Battlefield BC2 is a MMOFPS. But that sounds shitty, and we leave MMO out of the name. Which is why it's commonly called a FPS.
Same with SC2, it's a MMORTS. But commonly called a RTS. Since MPGH does not have a (MMO)RTS section it's placed in MMORPG. Because that's the closest to a MMORTS.