The human body has appeared in art from the beginning. Over time it has evolved from lines, to shapes. From paintings and then sculptures and that's when one culture took the human figure to another level. The Greeks created art on a human scale, they perfected the anatomy of the human figure as the way it should essentially be.
They were humanist they strived for improvement, to reach the maximum human potential. That is the reason for which their depiction of the human body is so perfect. The Greeks believed that the body, the athletic body was an aesthetic representation of nature, they were enamored with it and in our age and era we are too. The Greeks believed in Perfection, our society takes a healthy, beautiful body in high regards we are much like the greeks in the way we adore the pinnacle of what a human should be.
The Greeks and their representation of the human body is idealistic, im pretty sure not all greeks had a perfect physique. This depiction of flawlessness probably only encompassed the athletes and the soldiers. I believe that these sculptures was a way of creating an image for themselves sort of like advertisment. They could have also been a tribute to their gods. As gods they are perfection so their bodies must be perfect.