Usually it’s people who still buy the “if I work really hard to impress my boss I’ll get a piece of the pie one day” stuff.
(Edit: the opposite is usually true. If you’re too productive at your lower level job they won’t want to promote you because it would affect the numbers that management takes credit for. Saying in my trade: “if you get promoted it’s because you can’t insulate but the boss likes you”)

They see the union as a way for the undeserving workers to get a leg up while also removing their ability to be chosen by the boss to ascend to management