Most professional CG work is representative in nature. Concept art, matte painting, game art, 3D modeling⦠all require that the artist (for the most part) produce graphical assets that are instantly recognizable. As such, anyone looking to get a 2D or 3D CG job usually focuses on this kind of work, as it is what companies are looking for, and the kind of work that can be most easily used to gauge the skill level of a CG artist.
When learning art in general, itās common practice to start by studying and trying to recreate things that already exist (people, nature, objects, etc.) The reason for this is that there is something to compare the final work to, and a goal to strive for (accurate re-creation of the subject matter). Eventually, the artist may choose to explore abstract art, but having a solid understanding of the artistic process through representative studies helps that exploration on its way.
The early work of Pablo Picasso was representative⦠this self-portrait, for example. His later work, despite its far more abstract nature, still relies heavily on his foundational knowledge of form, shape and craft.
There are exceptions to the rule of CG art being representational, of course. My first (and, thus far, only) job as a professional (Lead) 3D CG artist/animator primarily involved designing, modeling and animating abstract interstatial scenes. However, even then, there were representational 3D scenes to create as well.
There was one abstract asset that I was originally slated to produce that just didnāt work in 3D: comic book-style motion lines. After about 5 failed tests, I suggested that they simply draw them in 2D. A couple of interns, pencils, rulers, and sheets of animation paper later, the effect was complete. 
The scene I initially created as an intern that caused the studio to give me a chance was representative (albeit simple) in nature. And, after the contract ended, I found that the footage on my reel did not open many doors to further 3D work, due to its abstract nature.
If these were forums primarily dedicated to painting or the fine arts, then there would most likely be a great deal more abstract art. However, being a CG forum, the above is (most likely) at least part of the reason you donāt see it as much as representative art.