I ask myself that every day, and yet here I am...
Honestly? Boredom. I think if anyone had something better to do, they'd be doing it. I would be, and soon, thankfully, I will.
Not that I don't have fun on the forums. I do. I enjoy getting to write and having an excuse to overanalyze these ultimately inconsequential topics. But almost no one needs the other people on the forum as anything more than a sounding board for their own ideas. No one has ever changed their mind on the Internet about anything, or been able to change anyone else's. The forum as it is is the exact opposite of what it was back in the Roman days: a place to go to exchange ideas and influence the state. Now it's a place to steel yourself against outside influence, discourage differences of opinion in others, and keep the industry from evolving- or it would, if developers paid any mind to forums, and I don't think they do.
You can, of course, make the argument that its all due to the need to seek out like-minded people, but that's officially bopkess. This simply doesn't happen on the Internet. If you wanted to congregate with other fans for congregation's sake, you'd be doing it in real life, and if you're too stunted to do it in real life, you aren't going to start on the Internet. Period.
The Escapist forum is one-half hate cult and one half circle jerk. Figuring out which half you're here for is really the only service you can do for yourself by being here.