You don't see a lot of open-source novels, either. For that matter, there's a community of people writing top-quality-for-the-genre games who generally release them for free, and they mostly don't release source to games, although they release source to libraries they wrote for games. (This being interactive fiction, where the modern authors have in many ways surpassed what Infocom did in their day.) There's a large extent to which not releasing the source for a game is motivated by the desire to avoid spoiling surprises or giving away the ending or the secrets.
I wouldn't be surprised if, in the future, we had top-quality open-source game engines, as well as vast libraries of art assets that may be freely used and tweaked in various ways, but games were generally not open source. (And I wouldn't be surprised if the engine/art/game architecture ended up being designed intentionally to make the game obviously not subject to engine or art license requirements, much as RMS's email is not subject to Emacs's license requirements.)