The answer is because men are mammals. And, in mammals, the development of mammary glands occurs very early in pregnancy, even before the setting of gender.
In other words, even before we know if the baby will be a boy or girl, appears the nipples that will later serve to drain the milk to feed their young, Or just to be the target of painful pranks in the playground.
"Both fetuses show nipple and areola at birth"
Only that while the men have just the nipple, areola and very rudimentary breast tissue (the mammary gland stops growing), in women, development continues until the moment of breast-feeding, in which specific structures for the production of milk appear.
In that moment, the development of the mammary gland is finished.
In theory, despite the poorly developed glands, men can produce milk when stimulated, due to disease or drugs or in situations of extreme hunger.
But they are rare cases. According to evolutionary biologist Jared Diamond, spontaneous lactation was found in prisoners of Nazi concentration camps and is related to a hormonal imbalance.
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