There is a big difference between a single player game and an MMO.
Not much -- games like Halo and Crysis have multiplayer in environments larger and far more complex than the small instances we get in STO. Besides the social zones, only Defera, Bajor, and Nukara have more than 5 players in them (ground zones).
I know it's "different". But the real difference is in the size (and budget) of the developer team and the maturity of the engine.
If STO were not so heavily instances and had seamless map transitions, I'd agree. But with all the zoning and small sandboxes, the MMO excuse doesn't fly as far.