Because kids with severe sensory issues can't even respond to their body's signals to the brain.
When we had a dinner with another family whose son had SID, the mom had to closely monitor his food consumption because he would literally eat until he puked. They don't get the response to the brain's signal telling them they're full or need to toilet or even an injury in some cases.
My middle son was pulling on his socks sitting too close to a space heater when I noticed the side of his arm was turning pink. By the time I'd pulled him away, the skin was blistered & he simply said 'hot' before trying to wipe off the blisters.