tuxedo-fish
Obnoxious Californian Stereotype
This is a very interesting topic. I don't think anyone knows for sure other than it's an evolutionary advantage, making livestock basically produce food for longer.
It's specifically that you suddenly have an option to get protein from your cow... without killing the cow. Killing the cow kinda defeats the purpose of getting long-term noms from said cow. Some people will bleed cattle for the same protein-rich purpose, but it's less common. And (fresh!) milk has fewer pathogens than blood, since it's intended to be eaten, and thus was/is a safe food source.
Plus, you can ferment and age milk, which extends its' life even longer. (Why is alcoholic horse yogurt a thing?)
I love cow milk. But I'm also becoming lactose intolerant as I age, so I'm leaning more and more on goat milk (better for lactose intolerance) and alternative (nut) milks.
Cashew milk is the closest, consistency-wise, to animal milk, so I'll go for that in smoothies and shakes. Pea milk (frex, Malk) is closer to flavor. And I do love me some chocolate Silk, but soy milk is super bad for people with ovaries, so I can't have it all the time. :<