With tech it's the other way around.
> Is it possible that there are factors outside of unionization that affect how these workers are paid?
Of course, there are tons of variables. But it's a fact that unionized workers make more than non-unionized workers in the same jobs, that unionized workers are much more likely to have health insurance, that unionized workers are much more likely to have paid vacation and sick time, etc etc.
1) Unionization is easier and more effective when defection is difficult (or impossible, as in some German unions) and heavily punished. As soon as defection pays off, it loses its value.
2) The set of high level executives at large corporations is small and cliquish enough that it is a de facto union, since they can convince boards to bid up each other's prices and punish/shun those who offer their services at lower prices.
The American Medical Association is a great example.