This is what is written in your link, "Only 13 percent manage to breast-feed exclusively for the six months that are recommended for a baby's health". If there was no formula do you think there would be a 87% mortality rate for babies?
It's a bit like how in the modern era where food access is abundant, it's seen as strange and unhealthy to fast. But somehow humanity survived before this access to food, at times not eating for weeks.
That source clearly states that only 13% do exclusively breastfeed their babies-- not can. It is common for women to "give up" on breastfeeding for a variety of reasons, but don't construe that as an inability to sustain their children's nutrition.
I think I understand the problem, but I think "able to provide breast milk" seems very open to interpretation. Babies are disruptive even before you have to pump or feed every 4 hours. But if you stop even a few times the body will stop providing milk. If your schedule doesn't support that (or you screw up occasionally) it will have a big effect.
However, that really means that you're not willing (ex a few percent who would have had dead children) to do what would have been required a few hundred years ago. Formula for a majority is a convenience... and has been since we used wet nurses and cow's milk.