Its amazing how the physiological bond is established between a mother and an infant. The species would not survive if this did not occur. Also there is no sound in the world that will awaken and draw a mother to her infant like the infant’s cry. It cannot be ignored. If an alarm clock were made that were effectively to mimic that cry, it would be too effective to use.
Speaking from personal experience, the baby’s cry also does a dandy job of waking up the father.