I couldn't pinpoint the time humans started drinking cow's milk, but we can assume that it started in the now idealized image of farm culture years ago. It was a product that could be produced in large volumes, unlike say, pig's milk, so it caught on, became profitable and then industrialized.
I think the more important question isn't why it started, but why people bother to continue? Sure milk has calcium, but that's not something we should fixate on. Dairy properties actually inhibit the absorption of calcium, and with alternatives like almond milk that have 50% more calcium available now, there's hardly an excuse not to switch.
Let's take a second to consider what's going into cow's milk these days. It's not that ideal farm culture, with a man in flannel milking the cow he knows by name in a spacious barn. It's a cow that's forced to be constantly pregnant so that it can be constantly milked. It's been fed growth hormones to produce more milk for less dollars, and fed antibiotics so it can survive on the cheapest diet. That's what you're drinking, secondhand antibiotics and hormones.
But even milk in its ideal, natural form isn't good for you. The purpose of cow's milk is to bring a baby calf to a full size cow. Casein, the protein found in cow's milk, reacts with the opiate receptors in the brain, mimicking the effects of drugs like heroin and morphine. It makes you happy, so yes, it's addictive. That's why you crave it. Nature intends this to bind a baby to its mother, not to bind us to the dairy aisle.
Milk exists like this for all mammals, so if you wouldn't drink gorilla's milk, there's no reason to drink cow's milk. So instead consider the alternatives; almond, coconut, soy, hemp, rice, hazelnut or sunflower milk. Really, the options are endless.