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Leon The Professional drank milk and he was good from what I recall, minus the whole being a hired killer…
I’ve always thought the trope was based on the idea of stunted development. Kids are heavily encouraged to drink milk, so films making a point of adults drinking it are indicating that there’s some part of growing up that they’ve missed.
It’s also not made for adults, it’s literally baby food for mamals.
While some parts of the world have been drinking it out of necessity to survive (in colder climate) most of the world haven’t developed a lactose tolerancy.
Adult milk drinkers are indeed freaks.