it’s because other mammals go through hormonal cycles (like having a mating season) whereas humans have evolved to have a ready structure for child rearing at any time (due to survival). It also helps that human society helped with this (like with wet nurses) as humans have evolved to continue producing milk for as long as their breasts are being suckled upon. To ELI5, breasts form during puberty and do not go away because humanity’s survival depends on having enough breasts to provide milk to suckle upon on, to provide adequate nutritional benefits for the tribe.
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