Earlier quoted context omitted.
Because unionized workers consistently have higher pay, better benefits and more rights. Next time you see the headline about some company laying off thousands of people while simultaneously making record profits and paying their CEO some number of hundreds of millions of dollars per year, ask yourself why that never seems to happen to unionized workers. Next time you see the headline about the "working poor" that ha…
Adding a union might net workers who join it higher pay in the short term, but who's to say that they aren't missing out on significant wage increases in the long term? You'll never see that counterfactual, and as a union gains momentum, it becomes increasingly unlikely we'll ever know how the workers might have performed without the union.
Economic theory, strategic behavior, and repeated observations through history (leading to the genesis if unions).