The capital flees almost in its entirety to non-union countries once unions grow in power, that's why.
Go ask the IATSE (film) folks how many jobs they're getting this year. (All their jobs left to developing economies in Eastern Europe and Asia. Cast gets flown out, but the crew are hired and based overseas.)
Unions contributed to the decline of US steel, US shipbuilding, US automotive, etc.
A worker can be both fungible and highly compensated. None of us are irreplaceable, and nor should we be. A union tries to put in artificial barriers where businesses need flexibility, and businesses really don't like that.