I actually didnt know that
The answer is apparently “because we’re actually able to eat it”
Fun fact: white people (specifically Northern European white people) have a genetic mutation that allows them to digest lactose even after weaning, which is abnormal for all mammals and also most humans. It’s theorized that because Northern Europe doesn’t get a lot of sun, an alternative source of vitamin D (like milk) would be a useful trait. It’s a very recent mutation that would only have happened after humans started domesticating animals like cows and goats.