Because babies eat from their mommies.
Milk is produced in the milk-producing glands in the breasts, called alveoli. Milk collects in pockets, or lactiferous sinuses under the areola (the dark circle surrounding the nipple). Regular and effective breastfeeding or pumping stimulates the production of breastmilk by causing the release of hormones from the mother's pituitary gland. Prolactin and oxytocin are the hormones of breastfeeding.