Nixon predicted it. Workers have asked for it. And businesses and governments have experimented with it for decades. The world has been talking about the four-day workweek for half a century, so what’s taking so long?

The idea pops up every so often in expectant headlines. Just last week, Microsoft Japan inspired a flood of stories after reporting that, in a trial, shortened weeks had boosted productivity by about 40 percent. Yet the four-day workweek is the flying car of labor: a profound advancement that has seemed just around the corner for decades.

“In America? I’m not expecting it anytime soon,” said Adam Grant, an organizational psychologist at the University of Pennsylvania’s Wharton School of Business, who made the case for a shorter workweek to business leaders at the World Economic Forum in Davos, Switzerland, earlier this year.

The reasons that a four-day workweek hasn’t yet taken hold are varied, Mr. Grant and others argue. Some barriers are institutional and some are cultural. And then there’s the most human reason of all: inertia.