Contact with center of palm, feet, sternum, throat, navel
I'm not sure if it is due to them being less touched due to their central positions especially so for the arch of the feet and the navel. But the center of palms do get a lot of daily contact, and the central chest bone is supposed to be a protection layer. So them being so sensitive makes no sense. My guess is that it is partly a survival instinct amplified by a fear of pain.