Goat milk is good. I’ve never had ass milk but it is apparently expensive. Some of the most expensive cheese in the world is made from goat and ass milk https://en.wikipedia.org/wiki/Pule_cheese
I hear Ass is difficult to milk
Also remember that cows aren't magic. It's only possible to get milk from a female cow for the same reason you can get milk from a female human. It's a female mammal which is lactating in order to produce food for its young. The cow was impregnated - it has no say in this matter - and the baby cow has been taken away (and in most cases killed) so that we can steal the milk instead. So that's... not great. Given we're apparently still murdering each other in considerable numbers in multiple wars this isn't on my top-10 "Worst things humans are doing right now" list but hey, it's not nothing.
The calf is either kept for a replacement or raised for beef. Too much potential value to kill them straight away.