One reason, using the Handicap principle, may be that, like the peacock's tail, not having hair is selected for because it is disadvantage. A female that can stand the temperature changes is obviously very healthy (or very capable of making clothing, shelter or fire), and hence would be a good potential mate.
Or because the female looks younger, more like a child than an adult capable of reproduction, they are more protected and cared for, thus giving their offspring an advantage.
Itâ€™s with some trepidation that I offer any thoughts on this issue, as I donâ€™t have anything approaching the levels of expertise or knowledge of some of the contributors to this thread. Itâ€™s also been a while since I read The Handicap Principle. However I seem to recall much of the early part of the book was devoted to the exchange of information.
The question then becomes, might it not be the case that the apparent hairlessness of humans offers greater visibility of the body itself, which better allows potential mates to make judgements about whether prospective partners are fit to carry each others genes forward to the next generation?
After all, youth, in of itself, does not necessarily render an individual the best bet in terms of ensuring that the next generation of oneâ€™s genes are in safe hands, so to speak. The ability to better determine how well put together a prospective partner is may be more valuable than how young or pretty they appear.