I think it's because people tend to stick with what's been proven to work in a full contact atmosphere. Boxing is simple and effective and is based on human body mechanics (as opposed to a bird, or lion, or cheetah, etc).
I also used gung fu style punches and strikes in addition to my boxing. I use my body in accordance with whatever flow of energy I'm giving (or am giving).
I think kickboxing is natural to most basic martial artists, it works, and is learnt relatively quickly. That is why we see it most often.
Though I've posed an offshoot of this question in the wing chun forum about fighting in a particular style should resemble that styles techniques and concepts. (So I complelety see where you're coming from).
"I don't know if anyone is known with the art of "sitting on your couch" here, but in my eyes it is also to be a martial art.
It is the art of avoiding dangerous situations. It helps you to avoid a dangerous situation by not actually being there. So lets say there is a dangerous situation going on somewhere other than your couch. You are safely seated on your couch so you have in a nutshell "difused" the situation."