Perhaps to look at it the other way, those with the tools/mindset to haggle and negotiate and prepare for a top career will end up at a place that offers top benefits and top salaries.
Which means the places that don't offer top benefits, top salaries, or places that otherwise mistreat their workers, will, almost by definition, retain mostly a workforce of employees who are either not top talent or are bad at negotiation (or some variation of this - e.g. the video game industry has some great programmers who work with awful treatment & pay for one reason or another).
If you're in this poorly treated group, unionization looks great. If most of your workforce is in this group, then most of your workforce will be eyeballing unionization.
Which means the places that don't offer top benefits, top salaries, or places that otherwise mistreat their workers, will, almost by definition, retain mostly a workforce of employees who are either not top talent or are bad at negotiation (or some variation of this - e.g. the video game industry has some great programmers who work with awful treatment & pay for one reason or another).
If you're in this poorly treated group, unionization looks great. If most of your workforce is in this group, then most of your workforce will be eyeballing unionization.