It seems like 90% of games released these days have some sort of dedicated multiplayer that's packaged with the game. Why is this? Because multiplayer sells. It might not sell much for some games, but these days gamers are smart. They know that if they are not getting a long, or at least engaging, single-player story, they want the ability to fall back on to something else to validate their purchase.
Multiplayer has been a main focus in gaming in general for a long time, going all the way back to the paper-and-dice days of Dungeons and Dragons. Having a friend join you on a quest or journey is just more fun, especially when both of you have the same end-goal in mind. In fact, the first video game that was created electronically, Tennis For Two on an old oscilloscope, was built for two players.
For me, multiplayer wasn't ever a main point in my decision on purchasing a video game. I was always just interested in the game itself. But as of late, I find multiplayer to be more and more a thought to run through my head, that is, when it looks like a sort of innovation. Just a few days ago I purchased Assassin's Creed: Brotherhood. Not only am I enjoying the fantastic continuation of the main story, but the multiplayer always has me coming back for more. Even though none of my other friends has AC:B, I still find much to like from this fresh take on playing multiplayer.
I know that their are some gamers out there who only make decisions based on if their friends will be playing it as well, and I'm finding it harder and harder to judge them for it. The benefits of cooperative gaming far out weight the negatives at this point. I might love the single-player narratives out there more than ever now, but I have to admit that it will only last as long as the developers want.
Just do me a favor, Multiplayer-Only gamers. Go play Ico.