Last time I heard that figure it had been asked why they weren’t and most replied they didn’t see a need - the company treats them well enough already.
Which could easily be due to a kind of Stockholm syndrome
Costco is known to be a pretty good employer.
Hmmm but that trend would likely hold true across the US, unless you think Costco has some specific brain-washing process :)
But as far as I know Costco is on the top ( or close ) of the list when it comes to employee happiness.