Almost all the statues and copies of statues we have from Greek times are gods and goddesses. The few exceptions are warriors and athletes. Those divinities were represented as humans — supremely beautiful human forms, supremely harmonious.
If you are going to sculpt a god you had better make him as beautiful as you can. And so you have to take a new look at the old flesh-and-bones contraption you thought you knew: now it was going to house a god.
When this happened; when artists began to render gods as humans — and not just king-humans in the robes of their authority — the human body became holy itself, a temple. All the supreme attributes appropriate to a god — harmony, dignity, serenity, awesomeness — were to be shown in their statue. And once a few generations had been taught to see the beauty in the statues of their gods, they couldn´t keep from seeing it in themselves — in the young men and women around them. The style of clothing of the times changed to highlight the bodies of the soldiers who fought half-naked the slavish Persian army all covered in tunics. It seemed to the Greek athletes and soldiers that they shared in the godliness of Apollo and Hercules.
Sculptors all through classical times (most were Greeks) continued to give or try to give their statues this divine harmony. The Romans never considered the body a temple but they liked the Greek treatment as good decoration, sexiness, dignity (for emperor statues), and so on, and ordered thousands of copies of the old statues for their halls and gardens. But by the last years of the Roman Empire Greek beauty was only a vague memory. And it disappeared altogether with the rise of Christianity. Though the Christians had plenty of good reasons to revere the human body, God Himself having become a man, they looked away from this world. The world was a place of suffering. Life was a trial-period, short, full of traps the Devil had laid to catch the weak and unwary. The human body was contradictory to its spirit, which was the real being, the immortal one. Beauty was gone.
Artistic beauty, unlike natural beauty, the beauty of Nature, is a luxury. It has to be cultivated. It comes after some of the basic necessities of life are settled — on a full belly, so to speak. It requires a whole system of patrons, artisans, connoisseurs — rich people who enjoy leisure and peace: in short: city life, commerce, civilization. In the fifth century, without those, the old classical ideal died.
See Biography of Beauty II and learn how it made a comeback.