Because kickboxing is the most natural way for the body to fight. It doesn't use any convoluted movements, doesn't mimic animals, and it doesn't follow preset methods based on theory.
That said, it could also be that the folks just don't perform well under pressure. A lot of folks are great at applying their techniques against little to moderate pressure, but when given REAL pressure they collapse either because their skill level isn't high enough, or because the technique or theory is flawed.
"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."