> Maybe barely post-cave people needed it to survive

Milk consumption is generally much more recent than that. Broadly speaking, while the ability for adult humans to digest milk probably arose several times independently in different populations throughout history, for most people of European descent it likely arose around the time the first Proto-Indo-Europeans were migrating from steppes of Asia to Europe. They were horse riders and herders it's likely their ability to digest milk from their herd animals gave them a huge advantage. While migrating it would have been an invaluable food source and likely gave them a physical advantage as well as some studies of remains from this time indicates they were physically larger on average than the native European populations they were displacing, which may help explain how they apparently dominated or assimilated the native groups so easily.

If you look at maps of lactose intolerance world-wide, the countries with the lowest rates generally overlap with countries that speak languages in the Indo-European language family.

I'm sure there's much more nuance to it than that, but lactose tolerance in adults is a relatively modern development.

> but in the modern world there is 0 reason to be consuming another species lactate.

I can think of at least two: many people think dairy products taste good and they are also good sources of nutrition.

There are many good criticisms of the modern dairy industry and our culture in general, but the simple act of using milk from other animals isn't the problem.