I wonder if it has to do with veiled’s rapid growth and short lives. In nature, they need to grow and mature as quickly as possible to reproduce before the dry season causes them to succumb. Long before I knew better, I followed an exotic pet store’s advice that my Grumpy would stop eating when she was full. Problem is that she never seemed full and never stopped eating or wanting more. She was downing like 2 dozen crickets daily and would have eaten more if I offered them. Their stomachs are like bottomless pits! Although we surmise they get no nutritional benefits from plant matter, what if they are just trying to fill those bottomless pits? In times of famine, humans have eaten all sorts of non-nutritious things to ease their empty stomachs. If one lives in a tree, your options are limited to leaves. Over time and evolution, it’s become innate in them to eat leaves.