One of the most common arguments I see repeated here when unions come up is the idea (not sure if it can be backed by evidence or not) that unions protect underperformers and stifle top performers. Combine this with the fact that most Americans believe they're above average intelligence [0] and you end up where a situation where most people believe that they personally would be worse off under a union.

I suspect that the HN demographic (and I include myself in this) is particularly prone to this sense of illusory superiority [1]. We're a demographic that is more likely than most to have grown up constantly receiving external validation that we're exceptional via good performance in school, getting into elite universities, occupying extremely high paying jobs, and so on.

[0] https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6029792/ [1] https://en.wikipedia.org/wiki/Illusory_superiority