There exist games in this world that don't need a multiplayer element. They're perfect as single player experiences. The campaign covers every need, making such a supplement unnecessary. These games on this list fall into that category. They all work well, or will work well, on their own merits.
Yet, they're games that would have benefitted from something more, had the time been put in. Each of these 10 games could have done something unique or special with a multiplayer option, and it's disappointing that we didn't get the opportunity to know there's something more there.