I think the biggest issue in making game open source is that most fan games are never finished and many of them don't have anything worth open sourcing as they are based on existing engines with minor alterations. They are abandoned, what means the authors don't want to put more effort into it and open soucing it is more effort. Especially with hosting because games can have some large assets, as well as some leftovers. Another option is open development, which is great because other people might join, but it scares away people who want more control over the project and don't want it to turn into an unsorted collection of random levels with random quality. Even with open development there's, again, the issue of large assets. With code/frameworks it's easy, just use git, but storage of textures, music, models, videos and Unity/UE4 project files - not so much. Gitlfs and hosting are additional expences and I doubt anyone wants to spend money on development of a non-profit game. Git also has an issue of people not knowing how to use it, fan games are often made by amateurs, after all. Personally I want to open the souce of Sonic Stranded (formerly known as Sonic Liquid Survival), but only after finishing the game. I'll do two releases - of the full game and cleaned up and streamlined framework with tutorial level and some level design tutorials. If people will make bad games from it, I will blame myself for bad tutorials, but I will be honored anyway because it would mean I helped someone have fun making a level. But I won't release it until I'm done with the game and some post-release bugfixes.