Why aren't hunger thirst and (the need to) sleep considered reflexes?
A reflex is an automatic and involuntary response of a living thing to the presence of a stimulus. A reflex is usually a movement, but it can be the activation of secretion in a gland.
Is it because there is no particular stimulus? Not having eaten, drank, slept for a while is not considered a stimulus?
Is it because you have to learn what thirst is, you have to learn that drinking solves the problem?