Where it comes from
Bodywork did not always wear clothes, Roman baths and Ottoman hammams worked skin against skin as a matter of course, and the draping rituals of the modern spa are Victorian inventions, not ancient ones. The naked format is less an innovation than a restoration: strip the nineteenth century's modesty apparatus back off the table and this is roughly what remains. Amsterdam, constitutionally unshockable, was always going to be the city where the restoration sold best.