When your life (and health coverage!) depends on your job and you don't have the energy or time to shop around for alternative jobs, you have little negotiating

When your life (and health coverage!) depends on your job and you don't have the energy or time to shop around for alternative jobs, you have little negotiating power. Your employer will not simply raise your wages just because productivity has increased without you finding some negotiating power to demand it. This is why labor unions are so helpful for the working class.