I am surprised no one has mentioned this yet. It has to do with the USA leaving the Bretton Woods agreement and the globalization results (aftermath) that came from it. And not to digress into whether Bretton was sustainable, because in many ways it was not. But it is no coincidence that productivity levels and wage growth remained consistent from the end of ww2 until 1972, after then they grew at alarmingly different rates.
"Since the early 1970s, the hourly inflation-adjusted wages received by the typical worker have barely risen, growing only 0.2% per year. In other words, though the economy has been growing, the primary way most people benefit from that growth has almost completely stalled." link
After 72 wealth generated from productivity no longer went to the workers, labors share of income has fallen from around 70% to around 50% now. This is from a variety of factors, the wealthy just taking more, but also due to outsourcing of jobs, mechanization, automation, and more competition in production leading to lower profit margins.
"Since the late 1970s, large wage gains have accrued to workers at the top of the distribution, and wages have been declining or stagnant for the bottom half of the income distribution.
Assigning relative responsibility to the policies and economic forces that underlie rising inequality or declining labor share is a challenge. International trade and technological progress have played significant roles, putting downward pressure on the wages of low-skilled workers. For example, as imports from low-wage countries made inroads into the manufacturing sector, job losses in the United States were substantial in some areas. At the same time, U.S. manufacturing has learned to produce more with fewer workers." link
Finally I will add that the growth of monopsonies (or near monopsonies) have had a large effect on wages. It is hard to leave one job for another with a better wage if that same conglomerate controls nearly all those jobs in your town. It is the same on the regional level (and national level) for most blue-collar jobs.
"New research by Benmelech, and Nittai Bergman and Hyunseob Kim of the National Bureau of Economic Research, indicates that the hidden culprit is what economists call labor-market concentration—too few employers competing for the same workers on a local level.
In other words, say a factory employee is dissatisfied with his pay and hears that a competitor across town is offering higher wages. He may switch employers. However, if there is no competitor to switch to—that is, if the local labor market is highly concentrated—then he must accept the wages at his current job.
“There has been a discussion in recent years about what happened to middle-class Americans,” Benmelech says. “We don’t say that we have the only explanation, but we have an explanation that is consistent and can explain the long-term phenomenon of stagnant wages...
...So many things have happened in the last 40 years—you have different policies, and the world is changing. But employer concentration seems to be an important factor,” he says. “It probably explains at least 30 percent of the fact that wages have not been increasing. And for economists, that’s a large amount of explanatory power.” link