There has been decades of propaganda about how unions destroy jobs in the United States and most software engineers have grown up in those decades.
I'm not trying to argue that Unions are exact right answer (perhaps something like worker's councils would be better) but the underlying issue is that collective action in the United States has been effectively demonized for a very long time (going back to blaming unions for our uncompetitive cars vs. Japan).
A neutral observation: The pro-union camp really needs some better messaging if they want any hope of overcoming these objections.
Nearly every pro-union discussion I see online or even politician speaking to a crowd feels like they're in full-on preaching to the choir mode, where they don't even consider how to address anyone skeptical of unionization. It's always presented as the obvious choice. Any skepticism or critical questions are dismissed as the result of consuming propaganda (like the comment above).
If the hardcore pro-union people want to get anywhere, they need to stop treating anyone with critical questions or skepticism as being misinformed or the victim of propaganda.
Speakers like Pete Buttigieg are a good model for addressing mixed audiences without alienating the other side right off the bat. Not everyone is going to agree with him, but he does a much better job of speaking to a mixed audience as a group of people with differing opinions than most.
It's almost like all of the forms of communication and media people pay attention to is owned by billionaires with a vested interest in promoting anti-union views.
As a group we're probably the most profoundly ignorant people on the planet when it comes to labor relations. We can't even reason about this because we (again, as a group) have practically no experience and even less interest in the subject.
The union issue vs. Japan is a perfect example because you only need to sit in the cars both countries were making at the time to understand why we were uncompetitive.
There has also been decades of corruption in management (see donations to ballrooms) and yet nobody is saying it will take decades to overcome management.
The problems with management aren't the result of any one person - it is the ownership class, their lack of any feeling of societal obligation, the lack of consequences for their actions, and their ownership of media and messaging.
I'm not trying to argue that Unions are exact right answer (perhaps something like worker's councils would be better) but the underlying issue is that collective action in the United States has been effectively demonized for a very long time (going back to blaming unions for our uncompetitive cars vs. Japan).