Milk purists are misguided or purposefully ignorant. Likely both.

an opaque white fluid rich in fat and protein, secreted by female mammals for the nourishment of their young.

This means if you're drinking an opaque white fluid that your mother didn't produce, it's not real milk. If you want to be a milk purist, you can only drink your mother's milk.

You aren't a cow, so if a cow produced an opaque white fluid for you, whatever that fluid is can't possibly be milk.

"Oh but we forcibly impregnate cows so that they make milk, their body thinks they are producing it for their young"

The definition of milk doesn't say "opaque white fluid produced by mammals for people to drink" it says SPECIFICALLY for the nourishment of their young. You aren't their young, therefore you can't call it milk.