Even in more open modding communities (like Minecraft) where these kinds of limitations are less common, it's really strange to me that most mods are not open source. By nature, mods can usually not be monetized in any way (without attracting the game publisher's army of lawyers) and are often developed by one person in their spare time (unsurprisingly leading to burnout and abandoned projects). So if you know you'll never make any money from it and that your time to work on it is very limited, why not open it up to contributions? Or at the very least, once you stop maintaining it for whatever reason, why not let others continue where you left off?