I believe that everyone has already made the response abundantly clear, however I still feel the need to put in my two cents (ten pence?)
Games that are popular are more often than not, games with an involving, clear-to-understand-plot, and gameplay that goes a bit beyond 'here is the Nazi/Russian/Afghanistan/Chinese/insert-cultural-group-here, and your gun. Introduce them.' That isn't to say games with multiplayer aren't good, but not to say that multiplayer instantly elevates a game to greatness. In fact, a game with multiplayer that doesn't require it feels forced and unfinished. Case and point, Resident Evil 5.
Now, resident evil 4 killed the RE franchise for me, it's no longer survival horror when you bust in a room and kill every monster/mutant by tossing a grenade then swinging about the ceiling with your machine gun like the bastard child of Rambo and Bruce Cambell, but I'm getting away from my point.
I'd actually say that many modern games go against the idea of multiplayer = win. Dragon Age Origins, The Dark Knight, Mass Effect 2, Assassin's Creed, Fallout 3. Games do not require the crutch of multiplayer to artificially enhance them.