Besides the reasons mentioned, another point I've heard from talented artist friends is that specific art assets have a tiny market -- Most games require the art to be customized a non-trivial amount to suit their game's atmosphere, theme, color schemes, etc -- Even generic things like zombies, fighter jets, rocket launchers, medkits, etc. So stock art assets tend to only be used in prototyping phases, and after that an artist is contracted to finalize the asset.
Since this is the usage of their products (early prototypes), artists can't charge the premium they deserve for the quality of the product.
The only way they can is they create super-specific but very cliche/popular artwork, like planet earth, Eiffel Tower, Statue of Liberty, iPhone, Desert Eagle, etc, which can actually be sold for a premium because they can be used as-is in polished final games. And then, only the best one or two of each item rise to the top of the marketplace and obviously noone needs the rest.
The Unity Asset store is a good place to look to get an idea for the economics of free/for-sale art assets. There are very few models/art assets that can be used in production, and the most usable/popular items are either tools, customizable assets like shaders, particle effects and materials, prototyping asset packs, or the specific items like I mentioned. And none of them can sell for very much.
From Unity devs I've heard the tools category makes the most money.
I make 3d models for games. In fact, I make 3d models for the game in the OP at MartianCraft. There's a vibrant community of game and 3d artists supporting eachother, giving out free assets, free help, free advice, tutorials ETC. I take issue with your broad assertion that artists don't support free culture.
When it comes to assets, it's tough to put together a library of free, or even purchased ones, because the style, poly count, texture resolution, texture mood, art direction ETC just won't be consistent across assets. We even purchased a couple of low-priority models and found that it took more time to fix them and bring them up to MartianCraft standards than it would have taken for us to design, re-topologize and texture assets ourselves.
Furthermore, almost everything needs to be designed and and built for specific layouts and proportions. If that pillar is a little too tall or not tall enough, you can't just scale it up as it will stretch the textures and look terrible. Things you can see and judge with your eyes are more difficult to piece together from disparate/inconsistent parts than invisible code.
Thanks for the detailed writeup of your experiences. What would you say would be different for a hobbyiest just learning 3D game development? Would free assets be more viable if you don't need a professional, polished look? Could you still get a playable game?
Ehhh, it's tough. What happens when you need a vertex moved or a poly count reduced? Visual assets really impact game performance in a way that requires bespoke work on almost everything. It doesnt help that the number of free game-ready assets out there is basically 0
I think that's selling content creators a little short - I think a lot of visual asset creators would love to contribute but are neither reached out to, nor sought after. It can be intimidating telling a graphics guy: just clone the git repo, put your assets in a fresh branch and we'll merge when were ready. You may as well be speaking Swahili to the poor volunteer! Making publically accessible list of needed assets, and a straightforward submission process for getting assets from contributors to programmers for integration might be the missing piece of the puzzle.
You're absolutely right that projects need to do a better job of welcoming non-technical volunteers. If some programmers have trouble contributing, it's even worse for everyone else.
In our defense (TD background), there are people that blend both, notably TDs ( http://en.wikipedia.org/wiki/Technical_director#Film ), sceners and game developers (especially indie ones) seem to fall into that category as well.
There are some free assets out there but they're mostly low-quality and the few production-quality ones are used in numerous low-quality games.