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.

You live a joyless existence if you avoid milk and cookies.

Is a hit of calcium and vitamin d when needed tho

I’d see smoking a cigar in a non smoking section and blowing it in someone’s face as more a telling sign of evil arrogance.

Milk is a good choice, gotta keep them bones strong

Some people are just so lactose intolerant smh

Meanwhile, hard liquor and alcohol are supposed to mean that you are normal and the good guy.

Also Homelander.

I grew up with milk being the main dinnertime beverage, and I still drink it quite a bit. My step-son's gf clearly did not, and clearly thinks it's strange, but tries to be polite about it. She's asked me if milk is my favorite drink, if I feel like milk goes with most food, and a number of related questions. I think in her family, milk is something some people put in coffee.

Why btw ? I love milk