Donald Trump, Ted Cruz, Hillary Clinton, and Bernie Sanders have made various promises about manufacturing in the United States — mainly that'll they'll bring it back from China to the long-suffering Rust Belt.

But as the New Yorker and 538 have pointed out, there's something missing from this pitch.

Mainly that while manufacturing may be returning, manufacturing jobs aren't.

Manufacturing jobs stagnated from the 1970s through 2000, and have dropped off since then. And yet production has been steadily increasing, save for a dip during the 2007 to 2010 recession.

So what's happening?

Factories are getting more efficient — and more automated. A 2015 Boston Consulting Group report found that 10% of manufacturing functions are fully automated today. BCG predicts that it'll be 25% by 2025.

As 538's Ben Casselman notes, manufacturing is a go-to campaign promise because is provides jobs with good pay to people without a college degree. Factory workers make $25 an hour on average; retail workers (who comprise more and more of the workforce) make less than $18.

The robots are coming for more than manufacturing. High-paying jobs (like those in the $350,000 a year range) are also at risk of being replaced by artificial intelligence.