(The one in TFA is an exception of course.)
You're creating, but your creation is totally dependent on the copyrighted work of someone else, and usually can't even be seen without it. "derivative works" bite that way.
The audience has already paid for the base game and there's not likely to be ways they can donate to you even if they wanted to. This is being worked on but still not quite there yet. Lots of the games customers feel they've paid enough already; which adds tot he usual "entitled user" problems.
People making mods tend to be non programmers passionate about an idea they had and wanting to make it happen even if they have to learn new things and extend themselves beyond their comfort zone. The social aspects of software development that encourage FOSS structures haven't occurred to them yet. Many will feel they're not "real" developers, this isn't "real" software, and thus those concerns don't apply to them or their work.
Since having a "mod community" is popular, many game publishers want to have one. They often have no idea how to foster a community and allow their inner demons to come out in their efforts to do so. If a mod scene has politics before the game is released, bail.