Because we are programmed to multiply our species as good as we can. A woman usually has only 1 child at a time after 9 months, so we better get to work :-)).
Of course nature could not forsee that our mighty man brain would put mankind at the very top of the foodchain and that man would pull modern medicine out of his ass and battle natures last failsafe device the pandemie.
Thats why we need religion and monogamy.
