I'd say that developing a game is often like writing a work of fiction. That's not to say that community participation isn't possible, but it does mean that it's not the same sort of interaction.
Open source developers can write KOffice, but it would be crazy to have KDE manage the documents that people write in KOffice. Wikipedia works on a very different development process from MediaWiki. Open source can provide the game engine and could provide the world and environment in which games could take place, but playing a game involves consuming content which is established as "canon", and that's inherently a process that's append-only once something is established.
Of course, there are also games which have little content and a lot of structure, like Street Fighter, Tetris, or Go. These work well for open source development. And there are sorts of content-heavy games where the tool support is sufficient that authors can take turns writing and releasing their games, which can be closed-source releases for portable open-source platforms. But there is, in general, something different going on from what happens when people write applications.