TLDR: Management has determined that current wages and working conditions are "good enough", and does not foresee that conditions might ever change to put workers in an even less favorable bargaining position. There is also the global race to the bottom to consider.

That this was written by "Diana Furchtgott-Roth, former chief economist at the U.S. Department of Labor", is worrying.

As a counterpoint, see the effect of unions in Denmark: https://news.ycombinator.com/item?id=28603398