If you don’t start nursing from the beginning, then your milk dries up. You can’t change your mind later. Cow’s milk is not an adequate substitute for formula

If you don’t start nursing from the beginning, then your milk dries up. You can’t change your mind later. Cow’s milk is not an adequate substitute for formula for an infant, but older children could switch. In addition to the other reasons mentioned by TrispusAttucks, I can tell you that breastfeeding is both very time-consuming and painful (because of plugged ducts). So I can understand why women opt out, especially if they have to work or have other issues.