It should also be pointed out that it's not just the money aspect. Managing people is difficult, managing people in a stressful situation is really difficult, add a dysfunctional culture (especially if that culture is "imposed" from above) is a pain in the proverbial. Which means managing in these situations is hard work.

Add on top of that that a largish number of managers just plain suck at their job, so being in a union can be useful to push back against either lazy or incompetent managers.

Because there are far more people who leave their manager than leave their job.