A lot of the more classical sculpture you see is just that – classical or influenced by classical scuplture. Classical sculpture grows out of the Greek tradition of kouros, which “is a statue of a standing nude youth that did not represent any one individual youth but the * idea* of youth.” (Getty Museum) Hair generally is associated with maturity, not youth.
Adolescents really aren’t old enough to get a good mat of hair going on his back or chest. I live on a co-ed floor, and it seems that a hairy guy in his late teens or early 20s is more the exception than the rule.
I think this and this is what JRDelirious means by “skin eruptions.” They’re beards, but still. Try rendering chest hair in marble.
As to the paintings, a lot of early paintings in western art were devotional paintings; that is, of saints and important figures in the early church. More emphasis was placed on capturing their saintly/virtuous qualities, rather than more hirsute/virile/sexual qualities, or even capturing a lifelike/accurate image. Also consider that in memorial art, such as Roman Egypt funeral portraiture, the emphasis was placed on capturing an accurate image of the face, not the body.
During the Renaissance, most paintings continued to be devotional, and the whole idea of youth and purity was still there. Commissioned portraits were generally of the artist’s patrons, and you really don’t want to piss off the guy who’s providing for your room and board by painting him as some hairy creature. Besides that, there really aren’t that many male nudes painted, and the majority of female nudes you see are from the waist up with the chemise open and breasts free. Most women don’t have noticeably hairy chests.
Later on, in the nineteenth century, there was this shame of body hair – it grows in “shameful” places, places that are “dirty”: groin, armpits, etc. It’s also associated with the start of adolescene and the beginning of sexual desire. (Hairy palms, anyone?) It’s interesting to note that Manet’s Olympia, which broke with the tradition of averted (i.e. shamed) eyes in nudes and odalisques, has no hair or nipples. Her breasts are perfectly smooth and white and there isn’t a hint of body hair. The increasingly stylized, informal painting styles seen in the latter half of the nineteenth and the early half of the twentieth century don’t call for the fine detail required for body hair.
It isn’t until the twentieth century that you begin to see brutal realism in portrayals of the human body, a sort of confessional style. Frida Kahlo is a good example of this.