3D artwork is a different ball park. You give a shape a texture, you make it transparent, translucent, glowering, you light the heck out of it, you create shadows and all that, but most of it is the interactions of 3d tools and constructs.
In skinning planes (like in AH) you're very much working in a set 2D environment. You want a transparent, translucent, or shadow-related look and you have to draw upon your own skills to figure out how to "paint" that effect into place, rather than constructing it piece by piece, and you have to make it look good in limited resolutions.
I'm not saying one is better or harder than the other, it's just that they are night and day in regards to practical application. Somebody might be great at one and not at the other. For example, I myself have take a number of 3D animation classes (up to the most advanced ones my college offered) and still have issues with applying textures and getting things to look right on a surface as compared to doing the same thing on a bitmap.