There's a lot more to manufacturing than "just" being a line assembly worker.

The factories have to be designed and built. This includes all of the manufacturing processes, equipment, tooling, automation, etc. All of which are done by reasonably paid, middle class engineers and trades.

Then you have all the 2nd order businesses that get stimulated. Energy must be provided. Mines, mills, refineries, etc. to make the raw materials. The packaging for the end products. Logistics for supplies and end products.

All of the value above used to be in the US but has been captured overseas for decades now.