monogamy and marriage developed to connect the father to the children
there will be witnesses that a baby came from a certain woman, but no witnesses that the father provided the seed for that baby unless he is married to the mother
it’s that simple
Life was hard and with no guarantee that there would be security or enough to eat for all.
Hunting, farming, fighting the elements, fighting wild animals, or fighting each other... men worked VERY hard just to keep everyone safe and fed.
WHY would a man want to struggle like that for children not his own?