I know that the general consensus seems to be that corporations despise unions because it inhibits their unrestrained profits, but I have a pet theory as to why American corporations specifically avoid unionization whilst other corporations in other Western countries seem to accept it as a reality of doing business.
I view it as a sort of prisoner's dilemma between two corporate boards. As a primer for those who are not familiar, there are two prisoners in solitary confinement who are given the choice of remaining silent or testifying against the other in exchange for being set free. The two prisoners cannot communicate with each other.
* If A and B each betray the other, each of them serves two years in prison
* If A betrays B but B remains silent, A will be set free and B will serve three years in prison
* If A remains silent but B betrays A, A will serve three years in prison and B will be set free
* If A and B both remain silent, both of them will serve only one year in prison (on the lesser charge).
https://en.wikipedia.org/wiki/Prisoner%27s_dilemma
In my example, the prisoners are presented with two options: unionize, or stay un-unionized.
There are a few tweaks I will make.
I view "prison" as the consequences of picking either option (there is no way to be set free). A short sentence represents the consequences of not unionizing: negative PR, labor lawsuits, and committing moral sins in whatever belief system you subscribe to. A long sentence is the consequence of unionizing: lost profits as wages and benefits are negotiated. A and B's investors will not approve of either getting a long sentence. Lastly, betrayal will be punished, not rewarded.
* If A and B unionize, they will both be sent to prison with long sentences.
* If A betrays B by unionizing but B doesn't, A goes to prison with a long sentence and B gets a short sentence.
* If B betrays A, A gets a short sentence and B gets the long sentence.
* If A and B do not unionize, both of them get the short sentence.
Given these rules, the only logical choice is to not unionize. If A or B unionize, they are hit with the long sentence. Corporations avoid unionization because their investors will punish them.
HOWEVER!
Imagine if A and B unionized, and every other prisoner in the prison also chose to unionize. The investors cannot punish every prisoner, because there is no one else left to invest in! Therefore, the long sentences (lower profits) are simply accepted because there is no way out - no way to avoid the long sentence.
That is why unionization enforced by government succeeds better than voluntary unionization. Investors are forced to accept the low results because they cannot choose to avoid unionized companies in favor of un-unionized ones. However, as long as the ability to avoid unionization exists, investors will do their damnedest to make sure that company boards work against worker movements. The boards must comply regardless of their personal beliefs, because they will lose the game if they allow the company to be unionized.