More Stuff, Less People Required
From a 2011 study entitled The Case of the Disappearing Large Employer Manufacturing Plants:
[…] the dramatic disappearance of large employers is neither mysterious nor surprising. Most of the missing large employers from 1997 are still open, only with fewer employees.
A stark example from the introduction:
[…] the steel plant in Gary with 30,000 workers in 1950. The plant is still in operation, but […] its current employment is down to 5,000. Remarkably, with one-sixth as many workers, the plant produces even more steel now than in 1950, as capacity has increased from 6 million to 7.5 million tons a year.
Simply put, we can now make more stuff with less people.