The same question could be asked about documentation writers. Most of the reasons given here as to why it is hard to get designers apply to documentation writers, too.
For open source games, the same question could also be asked for music composers, and again most of the same answers apply.
It might better to ask why is it so easy to get coders to contribute.
One of the proposed long term goals (still going through the approval process) for the Fedora project is to lower the barrier of entry for new members (Not a great link but one of the better ones that I could find: https://fedoraproject.org/wiki/Meeting:Board_meeting_2010-12...)
DISCLOSURE: I do work for Red Hat on the Fedora project but my interest in open source is also personal. I suggested this because it seemed appropriate and I'm familiar with it.
Thanks -- I'm going to apply! I have always had a Linux flavor on dual-boot on my PC for learning and tinkering. I'm more about end-user and customization documentation rather than admin/install, but hopefully they can use me!
For open source games, the same question could also be asked for music composers, and again most of the same answers apply.
It might better to ask why is it so easy to get coders to contribute.