Selective breeding. The cow as you know it, just like every farm animal, never existed in the wild. The caw was slowly domesticated from the wild Auroch.

https://en.m.wikipedia.org/wiki/Aurochs

Making too much milk is just a part of this process. Just like how a sheep will die from growing too much wool if let free.

Over the years, advances in genetics and nutrition have increased the production potential of dairy cows. We have done the same with all agricultural species and their respective products to make more product, more efficiently to keep up with demand. Milk replacer is made with standard nutrient values, so when it’s mixed to a fluid you know exactly how much protein, fat and vitamins are being given to calves. With milk coming off farm, you can’t guarantee each litre has the same values as there is greater variation. Some cows produce milk with more protein, some produce more fat and when it’s pooled together to be fed it’s harder to ascertain if the calves needs are being met or are in excess. Additionally, if calves are getting a higher protein milk one day or feeding and then a lower protein milk the next it can result in some serious digestive upsets that could be avoided when kept on a product with stable values.

For the same reason human mothers can often pump more than their baby needs and donate (or store) the excess. The amount you (or the cow) can produce depends on the age and size of the baby and how much stimulation the breast/udder receives.