If what you say is true, you should be able to convince shareholders/investors to require the workforce of the companies they own shares in to be unionized. There is no justification to back laws that limit the contracting rights of the employers. Such advocacy is based on a "I know better, so I will dictate your choices" principle.
And I would bet that companies that see their workforce unionize see a drop in market performance, across all dimensions: revenue, profits, innovation, etc. And I would make that bet in the market, and you would stand to make money off of me if you bet the opposite and were right.
But the labor laws as they stand don't allow these varying models to compete freely. As soon as a company's workforce unionizes, it receives regulatory protection from dismissal, and has the power to suppress the employer's contract freedom, by forcing it to forego negotiating with any party not in the union.
It's a situation that's ripe for abuse, and rampant abuse has occurred in such situations over the past century, as unions suck shareholders dry.