Unions aren't about more wages. Unions are about checking owner/capitalist/CEO power. Why does the CEO get paid 10-100x more than the lowest paid worker despite working the same number of hours a day? That's the question unions aim to solve.
This puzzle revolves around power. Power is the ability to execute force or inflict pleasure or pain. A CEO is clearly capable of painful force against their unskilled labor. A CEO can make it so someone has a harder time housing themselves, getting medical care, or feeding their children. But what power does unskilled labor have over their CEOs? Virtually none individually. So we see this reality reflected directly in wages. Unskilled labor gets paid the minimum because they have no power, and CEO's get incredible wages because they have the power to demand them.
This "agreement" is not mutual consent, but fundamentally exploitation. It's even more so exploitation when labor is sent to these poor countries.
Unions are the answer to the puzzle of "how does an unskilled laborer exercise power/prevent themselves from being exploited." Through collective action a union can cause the CEO pain and put a check on their otherwise unchecked power. In this way individuals are no longer powerless but have earned a seat at the negotiating table.
China did exactly this. They became exploited, nurtured dependence on the exploitation, and now use manufacturing's dependence on China as well as state level collective bargaining as a cudgul to execute their imperialist will on countries like Taiwan.
If you do not have the ability to commit great violence you are not peaceful, you are harmless. If you do not have the ability to execute power, you do not make agreements, you follow commands.
I personally don't understand how unions are not seen as American as the 2nd amendment. Unions are as important to preventing capitalist exploitation as guns are to preventing colonial/fascist exploitation.