Answers: Because they are supposed to drink milk exclusively. There are not many choices when your diet is made up of single one food item.
As an adult near lactose intolerance, I just don't drink milk, term. I have copious other choices that I make instead.
everything is more serious for babies not newly lactose intolerance. their immune systems and prtotective systems are not 100% and are not used to have to fight stale germs. thus, everything a baby encounter is a struggle to survive.