The reason Donald Trump cannot deliver on his promises to unemployed factory workers is that most of the manufacturing jobs being created or brought back from overseas are being automated.

Donald Trump has worked to make manufacturing jobs a theme of both his campaign and his presidency. He has long claimed that we are losing to other countries and that he alone can restore the old Rust Belt factory towns to their former glory.

Lately, he has been enthusiastically taking credit for every hiring announcement from U.S. factories, including the recent Ford plant expansions in Michigan that were in fact brokered by union leaders two years before he took office.

But there is a much bigger problem with Trump’s promises to bring back manufacturing jobs: Most of them never left. They are just not going to actual people.

Globalization and outsourcing have long been blamed by the general public and by politicians in both parties for the decline of American manufacturing. And there have indeed been many cases where manfacturers left America to exploit cheap labor and tax havens, but that is not the whole story. In fact, total U.S. manufacturing output is now double what it was in 1970. If globalization was the primary reason America lost factory jobs, production would have fallen as well.

The truth is that a much bigger driver of manufacturing job losses was automation. As factories invest in new technology, each worker can produce more per hour. Economists call this the “productivity of labor,” and it has been steadily rising for decades as more parts of the assembly line are mechanized.

On one hand, higher labor productivity means it is cheaper to produce and to buy everything we use in our daily lives — but the tradeoff is that factories have laid off more and more workers as machines take their place. One study found that between 2000 and 2010, 87 percent of manufacturing job losses were a result of factories using more efficient machines.

This is especially true in the U.S. auto industry, and it is why the new investments at the three Ford plants in Michigan, despite totaling a whopping $1.2 billion, will create only 130 new jobs. It is because Ford is “hiring” more robots than people.

A similar automation process is happening in the coal mining industry, and has put far more miners out of work than the Obama-era environmental regulations Trump is working to eliminate.

This is why Trump’s promises to bring back manufacturing jobs are empty. Even if he could somehow bring back every U.S. factory that was moved overseas; even if he found some way to do it that did not involve sweeping protective tariffs (which would kill millions of other jobs); even if he somehow got hundreds of extra new factories built as well — it would not come close to re-employing the millions of workers who were laid off over the years. We simply do not need as many people to run our factories as we used to.

But many of the people who voted Trump into office may not fully understand his unwillingness to address this reality. A new study from the National Bureau of Economic Research looked at Michigan’s election results and found that, in each county, the more workers who lost jobs to robots, the more likely the county was to vote for Trump.

We cannot have a national conversation about the future of the economy if the federal government cannot be honest about the root cause of structural changes in the industries they idealize. Just as he has on a host of other issues, Trump is selling snake oil on manufacturing. But he can only lie to jobless factory workers for so long before they realize they still do not have a job.