Being a "good writer" and a "professional writer" is a huge difference. Could any of us come up with a good story? Probably yes, huge conflict, itneresting characters - perfect. But that's not writing for a game. Writing is: what do all these NPCs in that remote village have to say? What about the actual dialogue, every single word has to be thought of etc. That is writing for a video game and many fan games prove that most people cannot do it (even in professional games, you will find horrible writing).
I don' say I could do anything for a game, and I say that because I know my skills and their limits. People who say they could just band together to make a game probably can't do it. Don't underestimate the effort. I was working with a small group on a German translation of the first Suikoden (which doesn't have all that much text) and even that was a horribly great job. And we weren't even creating the dialogues - we were simply translating it.