Because software copyright was and is a bad idea, and courts (especially the 9th Circuit) have generally carved out exceptions for compatibility-related copying in the software domain that absolutely would not apply to other copyrightable work that ships with the software. You absolutely can legally reimplement a computer program such as a game engine, but doing so to the story, assets, level design, or what have you would just be ordinary copyright infringement. As a result most game reimplementations either do not ship with any art, or ship with an entirely separate custom-made campaign to demonstrate the capabilities of the reimplemented game engine.