When you think of sports games, perhaps you immediately think of football, basketball, baseball, soccer, what have you. Golf might get a nod, as well as the various car-related events. However, at most, these diversions consist of teams with a rather small number of players -- nothing like the thousands to millions of users of your average MMO.
However, when you play solo, those other players might as well be bots for all you care. And if you're in a guild, it can be as small as your average basketball team. Playing against another team is essentially like PvP. And games like Mythos have instanced dungeons (for now, at least), so you never see another player outside of towns. This begs a larger question: if you don't actually see thousands of other players, is it really massively multiplayer? What's the cutoff number?