Unions resist innovations in the production process that make the jobs their members do more routinized and thus more efficient; unions keep jobs fun:
Unions also give teachers power to resist changes that make their jobs less fun. I think the teachers genuinely believe that these changes are bad; but I also think that they strenuously resist learning anything to the contrary. There is really good evidence for the benefits of direct instruction in teaching disadvantaged children. But direct instruction moves the teacher into being more of a technician and less of a creative professional. Ian Ayers talks about this in Supercrunchers, giving the example of bank loan officers, which used to be a skilled, prestigious jobs, and are now almost a clerical role. Doctors and teachers are resisting an attempt to do similar things to their jobs through, respectively, evidence based medicine and direct instruction.
The alternative to unions is not to have boring, routine, yet highly productive jobs. The alternative is for workers to find new more fun jobs when their old jobs are routinized ((Of course, the problem with this is that people’s identities are wrapped up in their job. “Get over it,” says the economist. “You’re an asshole,” says the sociologist.)). This will drive up wages in the routinized jobs (via a supply effect) and encourage employers to replace the routinized jobs with technology, like computers, that are good at doing routine tasks.