Graphically, it is just easier for the mothers to be nursing, and no have to have a whole new set of graphics for nursing mothers. Also, some parents don't really want to have that talk with the kiddos yet. I personally choose to beleive that there is a magical stork that delivers the babiesonce the parents have had a talk and decided gthey were ready to rear a child.
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Why is it that if someone tells you that there are 1 billion stars in the universe you will believe them,
but if they tell you a wall has wet paint you will have to touch it to be sure?