Paul Graham offers An Alternative Theory of Unions:
Oddly enough, it was working with startups that made me realize where the high-paying union job came from. In a rapidly growing market, you don’t worry too much about efficiency. It’s more important to grow fast. If there’s some mundane problem getting in your way, and there’s a simple solution that’s somewhat expensive, just take it and get on with more important things. EBay didn’t win by paying less for servers than their competitors.Difficult though it may be to imagine now, manufacturing was a growth industry in the mid twentieth century. This was an era when small firms making everything from cars to candy were getting consolidated into a new kind of corporation with national reach and huge economies of scale. You had to grow fast or die. Workers were for these companies what servers are for an Internet startup. A reliable supply was more important than low cost.
If you looked in the head of a 1950s auto executive, the attitude must have been: sure, give ‘em whatever they ask for, so long as the new model isn’t delayed.
In other words, those workers were not paid what their work was worth. Circumstances being what they were, companies would have been stupid to insist on paying them so little.
If you want a less controversial example of this phenomenon, ask anyone who worked as a consultant building web sites during the Internet Bubble. In the late nineties you could get paid huge sums of money for building the most trivial things. And yet does anyone who was there have any expectation those days will ever return? I doubt it. Surely everyone realizes that was just a temporary aberration.
The era of labor unions seems to have been the same kind of aberration, just spread over a longer period, and mixed together with a lot of ideology that prevents people from viewing it with as cold an eye as they would something like consulting during the Bubble.