The reason is that doing a complete set of artwork for a game is hard, and extremely time consuming. Most people, if they have the time, skills, and interest, will join a mod project, rather than develop something completely new from the ground up. Most of those mod projects subsequently amount to nothing due to poor interpersonal communications, inability to meet deadlines, real life getting in the way, etc.

So for a FOSS game artist, you're asking that a person be talented, dedicated, able to meet deadlines, not interested in the mod scene, technically adept (probably), good at working with others, and willing to work for free.

Then you have to find the same thing in a half dozen other people, some on sound, some on levels, environment, character models, etc.

Making a video game is a tremendous undertaking these days. Anyone capable of making a good game for free probably shouldn't sell themselves for that little.