> The Union sets the employees and the business up as adversaries and guarantees that there will be a contract negotiation.
> I don’t understand why people expect labor to be able to play hardball while also trying to benefit from a cooperative relationship with the adversarial partner.
I don't understand why people think it's not an adversarial relationship without a union. The workers are trying to get the most value for their labor, the company wants to get the most labor for its money. The difference is without a union the company has more power than you do.
Outside of the smallest businesses, most individual workers are disposable as far as the company is concerned. Their duties will be picked up by other employees and there's no rush to replace them. The company won't be substantially harmed by losing one worker, for the most part. There are of course exceptions, generally in small organizations or specialized skillsets, where individuals can be extremely hard to replace, but those cases are not the norm.
On the flip side most workers who lose their job have an immediate problem. Almost 2/3 of Americans live paycheck to paycheck. If they lose their job they have at most a few weeks before they're missing payments. No longer having a job with no immediate prospect to replace it poses an imminent threat to the stability of their life.
This gives employers a massive amount of leverage to push for the absolute minimum they can get away with before employees start saying "I'd rather starve", while employees have little to push back with.
A union attempts to balance the scales by turning the workforce in to a combined threat. No longer is the company negotiating against an individual, it's one or more entire classes of workers that they stand to lose and have to replace if they don't come to an agreement.