Everybody has a smartphone these days. If you don't, you're just pretending because you dropped your last one in the toilet. They're so pervasive they actually have a reputation for making us less social. Why talk to people at the dinner table when you can check your Twitter feed and post pictures of your food on Instagram? If you need to interact with everyone, just send a text. That's how we do it these days.
That anti-social reputation extends into mobile games. Phones don't make us want to interact with people, they're there so we can shut everything else out while we pretend to be super social by typing things into Facebook for our 1,017 "friends". They create this little world where you're the star, the big, bloated, gassy celestial body everything else in the solar system orbits. Firing up a game where you have to interact with people in real-time would destroy that.
Fortunately, not every game developer agrees that mobile platforms are destined for solo play. More and more games with co-op modes linger on the horizon. Many of them are ports, and many of them are sick with freemium features, but we've gotta start somewhere, right?
Actually, maybe the real reason why there are so few co-op games on mobile devices is because everyone's playing them in the bathroom. Toilets only support one person at a time. Maybe what we really need is a co-op toilet, then the games will follow.
We'll just leave you with that mental image.