Milk is protein! So :
Protein requirements for humans can range from as little as 40 grams to as much as 70 grams per day, depending on how much you weigh.
If you have 50 kg weight then you should eat 50–60 gram protein everyday.
If you have 60 kg then 60–70 gm protein.. and so on and so forth.
Hundreds of years ago we humans never used to drink milk but in the coming years we started drinking milk from cows and different animals just to meet our requirement of protein. We were always dependent on meat from the animals that we killed and we were eating their meat but in coming years we eventually settle down and we used animals milk to drink for our protein requirement because these animals were giving us many things. They were giving us their milk as well as these animals can be used as a clothing for us.
Eventually as a human being evolved and we started doing many different things we discovered a lot of different things from fire to sex.
but why do we drink milk from cow?
The simple answer is that that we were not able to hurt more, even not able to get the fruits or vegetables or cook so to fill our stomachs with something, so we relied on this milk and cows and different animals were easy to pet so we started drinking their milk as our source to food.