Because evolution is not an optimizing process? It easily gets stuck in local optima. Look at what a clusterfuck the 'design' of the human body is: from the susceptibility to all sorts of diseases, to the mechanical problems with the spine as support of our whole body weight, to plain weirdness like the appendix.

Your argument seems to revolve around the difference between humans and other mammals, with that I would agree - I don't see a reason it'd be different. But your argument also relies on the assumption that for other mammals the mother's milk is better. That's the whole question to be investigated - is it really so? If so, what is it in breast milk that is better? And once we figure that out, how can we put that into something that can more easily be manufactured and used, so that all women can get the advantages that breast milk provides? It's not like 'nature' is some magical thing that imbues mystical powers on matter. It's all physics and chemistry. We don't understand everything about it, maybe we only understand a tiny part of it; so that's why we have to do the research.