It could possibly be related to the structure/function arrangement of the proteins in the milk. I'm guessing that the structure of the human milk proteins match more of the active sites of the digestive path of a baby human. Has anyone ever looked at these proteins in that level of detail?
I know that my baby human intakes breastmilk and excretes a byproduct very different from that of the formula-fed babies I meet. The exact chemistry involved here is fascinating. I'll have to research a chemistry-nutrition specialist who can map it for me.