Cows are mammals. They produce milk for their young for a period after giving birth then stop, just like a human woman. Which means that for us to take their milk we have to keep them constantly pregnant.

Ask a woman (or think about it if you are one) how they would like being forcefully impregnated then having their tits constantly milked, year after year. As a bonus, the born kids are separated into girls to be milked in the same cycle and boys to be killed and eaten.