Writing is just another form of communication. Be it grunting noises, speech, or song, it has been in development for a very, very long time.
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:
http://www.youtube.com/watch?v=XrOgDCZ4GUo