They lack any maternal instinct in the sense that this creature needs to be taught to develop into a functional adult. To them, it's just another nestling crowded into the nest, tolerated only because it counts for gibs, and because it is small and weak enough to be easily exploited. That's why they be the same at 40 as at age 4, a lack of impulse control or realization that actions have consequences. That's also why they can't obey the simplest of commands. Their parenting is like raising tadpoles in a pond.