pray tell what exactly is the difference
I don't disagree, but I think due to our evolutionary history one requires significantly less practice than the other.
I'd love to be corrected on this if the opposite is true, that becoming a good athlete is just as difficult as or more difficult than becoming a good writer. I'm just not seeing it.
I won't comment on the difficulty of becoming a good athlete to that of being a good writer because it really is like comparing apples to oranges. That being said, becoming a good athlete can be incredibly tough both physically, and mentally.
Take a relatively simple sport like running. You will find a huge variation in the efficiency of joggers, even though it is a "natural" motion for the body. This is not even taking into account training, or nutrition. In fact, there are actually people who study the science of running:
This is like people calling skills "artistry."