Final answer:
Infants in Stage 3 fail the object-hiding task because they have not fully developed object permanence, a cognitive skill that allows them to understand that objects still exist even when they are not visible.
Step-by-step explanation:
Stage 3 infants often fail the object-hiding task due to the ongoing development of object permanence, which is a crucial cognitive milestone in early childhood. Jean Piaget's theory of cognitive development highlights how infants learn about their world through their senses and motor interactions, which includes achieving the understanding that objects continue to exist even when they cannot be seen or heard. However, before fully developing object permanence, infants will not search for hidden objects because they do not yet have the conceptual understanding that the object continues to exist once out of sight.
According to Piaget's stages, infants begin to develop object permanence between 5-8 months old. At this phase, when an object is hidden, such as a toy covered by a blanket, infants who have not fully developed object permanence will not try to uncover it, leading to the failure of the object-hiding task. This is because, in their cognitive experience, the hidden object no longer exists for them.