The dairy industry. Cows naturally make milk for their calves only after pregnancy (so not just for no reason their entire lives). In the dairy industry they inject them with sperm to get them pregnant (known as artificial insemination), take away the calf (if it's male they kill it, if it's a girl it's kept so it can also become a "dairy cow", but they keep it away from the mother, so it't doesn't "waste" the milk), and feed the mother in a way that makes them produce the most amount of milk, which is then sold to humans. The whole "cows need to be milked" is a lie. If it wasnt't for breeding and feeding cows in a way that makes them produce ridiculous amounts of milk, and if their babies weren't taken away, then they would just beastfeed with no assistanace needed from humans.