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.
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.
