First, how does "milk" work as a universal cross-cultural metaphor?
Try to name one culture any time in history or around the world now that does not use their own word for "milk" as a metaphor to describe at least one more valuable "thing" in their culture.

We're not able because:

#1) Milk reminds people in every culture about their mother.

And:

#2) Access to safe milk is a fundamental step in human survival - a process that starts when newborn babies in every culture require safe milk to survive at birth.

A story about the human survival instinct converted into action occurred when mothers in China became fearful of melamine-tainted baby forumula in 2008.
8 years later, Bain & Co conducted a study showing a pattern of how consumers in China still felt about Australia.

"Australia is a good place to go to get safe milk powder."
The Silk Road
This study reveals a larger pattern of which it is part - one of the largest patterns in human history.

"Humans constantly search for safe milk and all else this metaphor represents to ensure survival and growth."


This perspective helps create this economic theory:

"Demand from inside the Valeriepieris Circle is pulling supply from outside." (51% of people on Earth live inside the Valeriepieris Circle.)

A less technical & more fundamental description sounds like this:

"People are building roads to get milk like they built to get silk."

And a more matter-of-fact description is:

"People are building milk roads."
The Silk Road
A brief analysis of this pattern as a trend known as "The Milk Road Trend" starts here.