It's difficult to explain partly because over the last 10 years there has been more unskilled than skilled job growth which contradicts that narrative.
He indicated that dies bought from China and elsewhere are often 1/3 to 1/4 the price of dies made in America. They don’t last as long and the tolerances are usually looser than American dies. They may contain flaws that require some rework before being put into service.
Still, they are purchased in great volume, and he has trouble finding shops that can pay for someone with his expertise. He is overqualified relative to customer demand.
We can't expect to have a monotonically increasing salary throughout our lives; see: 2008 Financial Crisis for instances of folks with masters degrees taking minimum wage service industry jobs.