Games typically come with a lot of creative assets such as artwork, music, etc. that are copyrightable in their own right, but out of scope for open source lice

Games typically come with a lot of creative assets such as artwork, music, etc. that are copyrightable in their own right, but out of scope for open source licenses. Those assets could be released under a permissive license too (like Creative Commons), but a potential route for an "official", albeit open source game release would be to release the code freely, but not the assets. That could lead to clones with subpar assets, or -- just maybe -- someone/studio/whatever will create superior assets, improve on the game engine (as they're free to do so) and now you have a better game. Rinse. Repeat... The great thing about viral licenses like the GPL is that they effectively openly track the provenance of the original idea/implementation. If someone comes up with a really neat game (in terms of its concept and, ultimately, source code) and that gets built upon openly, the original copyright holder will still be credited. Plus, if the source freely exists, I think most would be more inclined to fork than to reimplement a clone from scratch (modulo platform support, I suppose).