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.
> Game mods often can't be open source as you don't have copyright on the original game files.
Don't most game mods simply patch the executable if there's no mod framework available? I would think that the patch engine and specific patches aren't violating the original game's copyright, simply because they wouldn't contain any of the game's assets or code (aside from maybe memory addresses and executable names for hooking)
There usually is a mod framework available, and in paradox games you often use the base game's script files as a starting point.
The point is generally irrelevant though, as a condition of uploading a mod to Steam or the game's mod system will be to give unconditional usage rights to the publisher which is incompatible with FOSS.
In that licensing case, a mod project could dual-license with both a license that conforms to the requirements for mod distribution and an open-source license, although this would complicate the efforts of any derivative code who might want to distribute.
That being said, mod authors seem weirdly defensive and secretive about their code and ideas; I think it would be a hard sell to get most of them to use an open-source license even if it were possible.
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.