A piece of software being open source doesn't mean it can't be copyrighted. Certain devs already sell their games without DRM, so it would not be unthinkable of them to make the source code public and allow the community to submit patches, ports to various platforms or whatever else while still retaining the copyright to that code, all the visual assets, sounds, story.
As for the software freedom aspect, games are entertainment. It's not as important for the user to be in control of toys as it is to be in control of their OS, browser, mail, etc.