"Milk does not keep for very long in a warm environment" (3:39) This was the explanation I was given, and it seems simpler to me: the addition of milk to coffee

"Milk does not keep for very long in a warm environment" (3:39) This was the explanation I was given, and it seems simpler to me: the addition of milk to coffee predates widespread refrigeration. Cows are milked in the morning, and fresh (unprocessed) milk will only be good for a few hours. Thus people only put milk in their coffee in the morning. This eventually becomes customary, and the custom is retained even after its original justification is removed (as with countless other culinary customs).