Your question only computes if what you say is true from your experience is generally true for others-- that is, if you can get internal training from athleticism. But I hear people saying that they are going far and wide to learn how to train internals, via exercises that are not present in athletics. So.. I guess whatever they are talking about can't be gotten from athletics.
People do all kinds of strange things. I wouldn't make any assumptions, for myself, based on the fact that "they" go far and wide to do anything.
So can you clarify? Do you mean why do we perceive internal skill as superior to the very different thing of athletic skill? Or do you mean "given that internal skill is learned within regular sports, why are things like shiko better?"
Why do you believe that internal is different then athletics? Why do you believe this is better?
Athletics teach you how to move smoothly and from your center. The modern study of athletic movement (as one would find in football, basket ball, track and field etc) teaches any of the things I can think of that are learned in "internal". The language is different but the lessons are the same. It's hard to go into anymore detail without further understanding your knowledge of sports training.