Because unions tether the best employees' compensation to the worst, and because company ownership (generally prohibited for union members) is a large fraction of TC for many SWEs, and because an additional layer of management and bureaucracy has negative value to most SWEs, few to none of the best engineers would join.
As a result, the union would be known as a place that caters to midwits and incompetents.
As a result, the union would be known as a place that caters to midwits and incompetents.