Because human capability requires resistance. If you wish to grow strong, you must lift the weights yourself. Suppose you build a powerful robot and send it to the gym using your membership card. The performance scores will look perfect, but your own muscles will completely atrophy. There is nothing in it for you anymore. An airplane flies and a bird flies, but the airplane flies in a way that is of no help whatsoever to the bird. A tool can perform a task without strengthening the user. The machine's capability does not transfer to the human.