Have you ever considered that you one day could be that "under performing" employee grasping on to their livelihood during a period of poor health, family problems etc?
Management doesn't pursue these people because of the effort involved with dealing with the union.
I'm open to a system that allows all of these things to happen.
However - applying that system on a micro scale (one workplace at a time, through unions) introduces way too much friction between employers and their employees, employees and their peers, and employers and unions.
It also causes an internal conflict for the employee - where they have to mask their lack of contribution by busywork, ultimately feeling unfulfilled in their life.
Unionizing all workplaces is a sub-optimal means to distribute wealth.