However, workers can undertake actions to increase their bargaining power and thus wages, as can any other supplier. [...] is no different than any economic transaction
It is different, because of the official governmental support worker-unions enjoy — instead of being treated with the anti-trust laws, like any other entity working to raise the prices of what its members are selling.
Except, unlike a group of producers acting in concert to exert market power; an employer still has many other options for labor. They can outsource, refuse to sign a contract and bring in replacements, move to another non-union location; unlike a monopoly where there is no other source of the product. Granted, those are not easy things to do but hey still are viable competitors to a union workforce. The government has intervened in the workplace in many ways, sometimes to the workers favor (unions, labor laws) and other times to the employers (non-competes, right to work laws,letting bankruptcy abrogate contracts and pension liabilities).