Cows are programmed to have a calf every spring to match grass growth.
Nature may have programmed it for every spring.
But modern factory farming has other ideas.
After giving birth, they lactate for 10 months and are then inseminated again, continuing the cycle. Some spend their entire lives standing on concrete floors; others are confined to massive, crowded sheds, where they are forced to live amid their own waste.
Cows have a natural lifespan of about 25 years and can produce milk for eight or nine years. However, the stress caused by the conditions on factory farms leads to disease, lameness, and reproductive problems that render cows worthless to the dairy industry by the time that they're 4 or 5 years old, at which time they are sent to be slaughtered.
Cow milk has one purpose, the nutritional needs of calves, who have four stomachs and gain hundreds of pounds in a matter of months.
Cows produce milk for the same reason that humans doto nourish their youngbut calves on dairy farms are taken away from their mothers when they are just 1 day old. They are fed milk replacers (including cattle blood) so that their mothers' milk can be sold to humans.
Besides humans (and companion animals who are fed by humans), no species drinks milk beyond infancy or drinks the milk of another species.
If we need to drink milk as nature intended, we should be looking for jars of human milk at the supermarket.