One of the big changes in the post war era was that immigration was massively opened up in 1965. From 1924 to 1965 the US had very restrictive immigration laws, which led to labor shortages, which allowed unions to become strong, rising wages and the expansion of the middle class. Since 1965 we've had declining union participation.

This is simple supply and demand. If you restrict the labor supply, the value of labor increases.

The same thing was observed after the Black Death, which killed off 30 to 50% of Europe's population. There were labor shortages, which increased the bargaining power of labor, and increased wages.

It's really funny US companies suddenly start pretending they don't believe in supply and demand when it comes to labor.