The spectre of any large worksites becoming unionized keeps significant amounts of manufacturing out of the US too.
Increasing bargaining power is a Red Herring to convince the population to restrict people's rights to freely contract, for the benefit of unions. Everyone already has infinite bargaining power in a market economy, which is why the only negotiating leverage a prospective employer has to convince an individual to work for them is to offer them better employment terms.
A society where voluntary interaction is not restricted by labor laws sees more rapid economic development, and with it, wage growth, which is why Hong Kong and Singapore massively closed the wage gap between them and the Nordic countries over the last 50 years.
It doesn't matter how much value you generate for The Company. If you don't have strong labour (eg. via unions) then there's absolutely no reason for a company to compensate employees fairly. Companies are not only happy, but DEFAULT to raising prices AND lowering wages. Max profit.