Game mods often can't be open source as you don't have copyright on the original game files.

Another problem is that game mods are often developed in private repos to prevent ideas and assets being "stolen" by other projects. There are no lawyers to protect your IP in the non-commercial world.

It's taken decades for the commercial world to even vaguely become OK with source available, let alone open source, we can't really expect teenagers and young adults who weren't trained as software developers to be comfortable with it.