The U.S. economy is besting expectations for job growth, and the unemployment rate is at its lowest in several decades — but the other side of the story is that millions of jobs out there just aren't good enough.

Why it matters: Almost half of all American workers are stuck in low-wage jobs that often don't pay enough to support their lives, lack benefits and sit squarely inside the automation bullseye.

By the numbers:

There are 53 million U.S. workers — around 44% of the total workforce — who work in jobs with a median hourly wage of $10.22 and median yearly earnings of $18,000, according to a November Brookings study that examines low-wage work.

Around a quarter of low-wage workers are the only earners in their households.

"The storyline from the jobs reports is the result of looking at an incomplete set of measures," says Martha Ross, co-author of the Brookings report. "We do have to look at the quality of the jobs we are creating."

The big picture: For decades, the job market has seen steady polarization, Jed Kolko, chief economist at Indeed, says. There's been growth in high-wage jobs in tech and finance in big cities, and there's been a similar surge in jobs at the lower end — but the middle has hollowed out, primarily due to the collapse of manufacturing.

The labor market is tightening, which ought to push wages up, but that's not necessarily happening, says Kolko. While pay for low-wage jobs has been increasing in the last couple of years, that growth is not keeping up with inflation or the rise in housing costs.

On top of that, millions of low-wage jobs — such as in retail, food service, home health care or the gig economy — don't have robust benefits packages (if any) and don't have predictable schedules. "All of this is part of what makes a job a good job," he says. "The wage growth for lower-wage industries is encouraging, but there’s lots to worry about."

Low-wage work is more pervasive in parts of the country that have been left behind by the winner-take-all cities on the coasts, Brookings found.

And there are a number of cities where low-wage occupations make up the majority of jobs: Las Cruces, New Mexico, and Jacksonville, North Carolina (both 62%); Visalia, California (58%); Yuma, Arizona (57%); and McAllen, Texas (56%).

This will be an increasingly important question for local leaders, says Ross. "What does it mean when almost two-thirds of the jobs in your region are low wage?"

Minorities are disproportionately impacted by low-wage work. Per the study, 54% of black workers and 66% of Hispanic workers are low wage, compared to 37% of white workers.

What to watch: Polarization in the job market is projected to get worse. At the same time, low-paying jobs in the retail and restaurant industries are among those with the highest automation potential. Says Ross: "The question of these low-wage jobs disappearing or changing means we have to think about how to support these workers. I think, as a country, we are terrible at doing that."

Go deeper: An uncertain future for workers