In my experience (contributing to various open source projects), skilled graphic artists are greedy pigs and rarely do anything for free. Lots of programmers release their work for free, improve and fix it. But artists are invariably going for where the most money is.
Considering this, it doesn't surprise me in the least that often only source code is released. You get emulators and engine reimplementations for countless old games, but a release of a full game is rare.