The idea of referring to things that look like milk as milk is much older than those examples. Consider, Milkweed which produces something that looks like cows but is a very different substance. https://en.m.wikipedia.org/wiki/Asclepias

Not to mention the Milky Way.

Not all proteins are the same.

Also keep in mind that it took humans some time of evolution to be able to digest cow milk.

Not really.

All human babies have a much greater propensity to be able to digest lactose than adult humans.

Whey and casein are the dominant proteins, and lactose the dominant carbohydrate, in both human and cow milk.

Humans and cows aren’t that different. All mammals have quite a lot in common.

And that the majority of people on earth are still lactose intolerant.

That’s a relative issue: “People are usually able to drink at least one cup of milk per sitting without developing significant symptoms, with greater amounts tolerated if drunk with a meal or throughout the day.”

This is because most people produce some lactase it’s more a question on quantity of milk consumed.