Both nominees are explaining how they’ll bring back manufacturing jobs. “We are builders, and we need to get back to building things!” Clinton told an audience in Michigan last week, explaining how she’d bring hundreds of thousands of jobs to Michigan. Trump told a crowd in Columbus that he’ll make Ohio “a manufacturing behemoth…. We’re gonna bring your jobs back, we’re gonna bring companies back.”

Of course we know why they say this. Michigan, Ohio, and Pennsylvania are among the most important swing states in this election, and they’ve lost millions of high-paying manufacturing jobs.

Many voters – especially old ones, who are most likely to vote – are susceptible to a fantasy of bringing back the good old days. Clinton told her Michigan audience, “You should be able to learn a skill, practice a trade, and make a good living doing it.” Just like in the 1960s. If Trump renegotiates NAFTA, said a retired rally attendee in Pennsylvania, “we can start bringing back industries and make televisions, we can make refrigerators. I saw them leave. Why won’t they come back?”

Watch: The Big Problem with Hillary Clinton’s Job Creation Plan

They won’t come back because the world has changed. Manufacturing jobs were about 40% of U.S. employment in 1940; today they’re less than 10%, but we manufacture far more now. It’s the same worldwide – in the U.K., Germany, Australia, Brazil, India, China, and many other countries, the manufacturing share of employment is falling. Everybody’s making more stuff with fewer people, and the trend isn’t going to reverse. Suggesting that it will is inexcusable.

The obscure event that brings the lesson home is on Friday, when the results of the U.S. ProFarmer Crop Tour will be announced. Never heard of it? That’s not surprising. Since fewer than 2% of U.S. workers are in agriculture, hardly anyone pays attention to estimates of Midwest crop yields. Yet those 2% produce so much food so efficiently that obesity has become a U.S. health crisis.

Sign up for daily insights, updates, and opinion on leadership and leaders in the news at the Power Sheet.

Back in 1820, when the industrial revolution was getting underway in the U.S., 60% of workers were in agriculture, and conventional wisdom held that manufacturing could never be the basis of an economy. That view was mistaken, of course. As the economy was transformed, that 60% dwindled to 2%.

Now the economy is being transformed again, and it’s irresponsible to deny that reality. Would America be better off if we went back to the good old days when 60% of us worked on farms? Obviously not. The larger point for all leaders is that the world never stops changing, and the leader’s job is to embrace the new reality, explaining how it can bring a better future, not a worse one.