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Found this great post today that explains this mystery: http://alicebessoni.tumblr.com/post/117118471190/why-wont-artists-accept-revenue-share-royalties
I got some non-exclusive graphics packs for sale over at: https://www.gamedevmarket.net/member/dhondon/ Check it out!
A useful guide I found on reddit.com. If this is your first time contacting an freelancer, this will be very enlightening: http://www.reddit.com/r/gameDevClassifieds/comments/2b4b5f/the_guide_to_hiring_an_artist/