laziness. i wish i was kidding. there are almost no women who are actually unable to produce even today. on your last point, the milk production doesn't begin in earnest until the newborn is sucking on the breast. women only produce colostrum during the pregnancy and won't begin producing milk unless someone sucks it out of her tiddies. so if you never start breast feeding you never get the discomfort of overfilled balloons.
Denmark