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.
Actually, it’s a mutation that happened three separate times in three different populations. Northern Europe, the Middle East, and a select population in.. East Africa, I think?
One big explanation for why these populations have lactase persistence is livestock and famine. If you can drink milk when there’s a famine and little else to eat, you’re way more likely to survive than those that can’t.