I guess it has something to with amygdala, that's a part of brain responsible for fear. Imagine you're walking on a ground and you feel perfectly safe. Now you're walking one foot above the ground on a narrow pathway, you're aware that if you misstep a bit you fall down by one foot. It's no biggie but it might cause a strain ankle or something, so you're not in panic but slightly aware. If that narrow pathway thousand feet above the ground you're panicing you want to get down and hug the pathway and praise the Gods to save your life. Amygdala assesses the situation and set the appropriate level of anxiety based on situation. Now imagine that your amygdala is binary. It only sees zero inches above the ground and anything higher is high enough to panic. It sees no difference between one foot about the ground and a mile. Above the ground is above the ground no matter how high.