According to Jewish law, the milking proces needs to be supervised by a Jew to make sure the milk is only from kosher animals even though today in most countries Dairy Farms are reliable we still keep this law just like so many of the other laws in Judaism that the circumstances for their reasonings might have changed, we still keep them fully since there is a spiritual element to every Mitzvah which gives the Jew the ability to connect to G-D through that Jewish law that spiritual element never