That's not been true for decades. Have you ever worked in any of the numerous fields of manufacturing? First, our production plants are highly automated, which requires a tremendous degree of 'labor' skill to maintain and often even just operate these machines. Second, you have heard of SPC, Arthur Deming's statistical process control, haven't you?
As implemented here and in Japan, The workers on the line are incorporated into the quality assurance process, and are carefully to guage the production tolerances. And if they deviate unacceptably, to stop all production until finding a fix.
It is often these same workers who come up with the fix. Not management. This is highly skilled in most manufacturing, and requires a deeper understanding of things than you seem to appreciate.
And then on the macro-economic level, you have to realize that nothing has repealed THIS:
Chart Source: U.S. Commerce Dept.
Manufacturing’s use of intermediate goods and services in its production process means that it generates substantial economic activity at the intermediate level. This is called the multiplier effect, and it turns out that manufacturing’s multiplier effect is stronger than other sectors.
Specifically, every $1 of a manufacturing product sold to a final user generates an additional $1.43 of intermediate economic output, more than half in sectors outside manufacturing. Manufacturing’s multiplier effect is greater than any other sector and far greater than that of the service sector, which generates only 71 cents of intermediate activity for $1 of final sales—half of the additional intermediate output generated by $1 of manufacturing final sales.