A theory: The ability to capture the rewards depends to a large extent on political power within the organization (i.e. rather than merit and market value). Management has great political power, naturally; that's why in some organizations even failing managers get massive bonuses, golden parachutes, etc.. For workers to have power they need to organize themselves, and that is what unions are: Political power for workers.

People complain that unions are corrupt, which of course some are and to different degrees. That's also true of managers and anyone else with political power.