Milk consumption after weaning is not common to people all around the world. In fact, most of the world's population doesn't consume cow's milk and dairy products not just because it's not a part of their culture but also because of higher prevalence of lactose intolerance (inability to digest milk sugar - lactose - in adulthood). Please see our infographic to learn more about the issue.
Ethnocentrism means judging another culture solely by the values and standards of one's own culture. And in the case of milk, this has had many unpleasant consequences such as milk powder sent as food aid giving people severe digestive problems and schools putting pressure on parents of children who might be lactose intolerant to give them milk.
The prime example of how harmful ethnocentrism can be is this excerpt from Professor William H. Durham's book, Co-evolution: Genes, Culture and Human Diversity. (W.H. Durham is a Professor in Human Biology at Stanford University)