Actually, it used to be a thing where men were relatively skimpily dressed too. Conan of Cimmeria goes barechested, with leather boots and loincloth. John Carter of Mars wears less, and depending who draws Elric of Melnibone, he is often wearing not much either. Trazan of the Apes as well. A lot of the "naked fantasy" look is from artists like Frank Frazetta, Julie Bell, and Boris Vallejo. It's just generally that it seems to have fallen out of fashion from the late seventies.
if you want a humorous example, go watch a clip of the old fantasy movie Zardoz. You can see the esteemed Sean Connery in a one piece man swimsuit, with thigh-highs and a mustache. It's funny now, but that actually was typical fantasy/sf house style for artwork at the time.
