There are a few reasons. I live in Michigan next door to a dairy farm so it’s totally understandable not to know. The first reason is the treatment in factory farms. Obviously this is a problem, but there are still huge issues with objectively better farms like the local ones in Wisconsin. A cow, like any other animal, only produces milk for its offspring. Female cows are kept perpetually pregnant from bull sperm that was electrically ejaculated. The farmer fits their arm in the female cows anus in order to impregnate them. When the baby is born, it is immediately separated from its mother so it doesn’t drink the milk. The males either are turned into veal or thrown out. The females are kept to be bred. Obviously the cow can only handle being pregnant so many times, so they stop being useful when they are about 5 even though they naturally live to be 25. Every cow is eventually killed early in a slaughterhouse. It is required by law for every cow to be slaughtered because it is the only way the dairy industry can be sustainable. These are all standard practices and happen in all of the most “ethical” farms. I hope that was helpful I’m glad that you want to hear more about these things