That's because nobody wants to work those jobs anymore, and for some, work at all. There are 10K jobs in manufacturing open in my community college district alone, and we have a great program to get people qualified for these jobs. These are jobs that start at $50+ (read: living wage), with just a 1 year certificate. There are a ton of jobs open. We need people to come get them. The people are out there. This I know.
This isn't exactly true. Millions of manufacturing jobs were lost in the 90s and 2000s and those are never coming back. In the 1960s about 1 in 4 Americans had a manufacturing job. 25%. Today it's less than 1 in 10 at around 8%. The US has lost 6 million manufacturing jobs since 2000 alone. We used to manufacture everything under the sun here. We don't anymore.
Take the state of Michigan alone. From 2000-2010 it lost 48% of all it's manufacturing jobs. In the 1960s the city of Detroit was the 4th most populated city in the country with around 2 million residents and it was the richest city in the entire nation. All of that built on the back of manufacturing. Now the city of Detroit is destitute. A true American Economic Horror Story. There's less than 670,00 residents, the city went bankrupt not long ago, it's the most dangerous city in the US, and it looks like a fricken war zone with abandoned factories, homes, and buildings everywhere.
Automation/robotics and out-sourcing have devastated most US manufacturing jobs. Automation not as much as out-sourcing, but it's played a part for sure. The US is actually lagging behind most of it's competitors countries in terms of automation btw. South Korea, Germany, Japan- all have far more robotics in their manufacturing plants than the US does. Some of those countries by a factor 5x. And 78,000 plants were closed in the US from 2000-2014 alone. The robots need somewhere to work too. If automation played such a big role they wouldn't be closing that many factories, they'd be retrofitting them with robots.
Clinton signing NAFTA into law in '93 and pushing through permanent normal trade relations with China in the last year of his presidency- which set the spring board for China to join the WTO in 2001- lead to massive layoffs in the US and shifting production to other countries where US multinationals could pay employees in other countries slave wages and pollute the environment all they wanted and not have to deal with pesky things like fines from the EPA thereby boosting profits by saving shit tons of money on labor and also saving money by not having to deal with fines and regulations from the US government agencies like OSHA/EPA.