A single-player game does not need to be challenging to be fun. It doesn't actually have to be hard to complete. A single-player game can present an interesting storyline in ways that a multi-player game cannot (or, at least, has not yet).
In a single-player game you can develop characters and settings. You can explore a world. You can show the consequences of your actions. You can have a whole story arc.
My thoughts exactly. Pretty much every BioWare game ever (especially Dragon Age or the Mass Effect games) is about story and consequences; the gameplay is not terribly difficult, even on the highest difficulty settings. Same thing can be said for Bethesda games, like Fallout 3 or Oblivion; not 'impossible' to beat by any stretch, and far more about exploring interesting new worlds. All of these examples are games that did extremely well without ANY multiplayer 'tacked on', as it were
Multiplayer has its place, certainly. Every now and then I just need to put in Left 4 Dead and massacre some zombies with a couple of friends. But it's nice to separate myself from that sometimes and dig into a truly engrossing story.
I think the main point here is that nobody has found a good way to merge the two, so we are all stuck with either: a) single-player-focused games with deep and moving storylines / settings and a forced, artificial multiplayer tacked on or b) multiplayer-focused games with forced, artificial story / characters tacked on.