Why are bare chested boys used in photography?

Because the photographer likes looking at bare chested boys and/or feels his target audience would like to do so.

Or, possibly, the photographer might be trying to get some kind of contrast between skin tone and background color because he or she feels it's visually interesting, and bare chested boys provide a relatively high amount of skin tone area without the controversy involved in photographing bare chested girls. Particularly if the chests in question belong to those under the age of 18.