When it comes to games, I honestly understand it a little less. With fan films, I can see that gets interest and people willing to help out more easily. With games, for the same amount of work you can do your own thing and actually try to make money. And then you can get a job in the gaming industry, realize everything is awful, and become an incredible alcoholic or smoke meth.
Well, I somewhat agree there. Game development, or what little I know of it, is very labor intensive. All I can imagine is if a developer of a fan game puts that kind of effort into such a project, it's either they are an uber fan, OR, they are basically developing a calling card of sorts, a demo tape if you will, for future work. I agree though, it is better to put that kind of energy into your own work, rather than someone else's IP. Also, if for no other reason, why risk being sued?