Employees have an inherent need for loyalty to work and face considerable risk just for following their true worth to new companies and situations.

This makes it impossible to adequately take employers up to your real market worth as a long term employee. Too many of your coworkers will accept 50% of their worth rather than risk stress and a crises if a change falls apart. So as long as the employer only needs a kernel of long-term employees only a union has the necessary leverage.