> I don't want another bureaucratic layer that tells me what to do, and run by yet more HR admin-types.

That new bureaucratic layer would be designed to benefit you, and if it were to stop doing that and suddenly no longer served the interests of it's members you'd have the power to replace the leadership of that union or to leave it and start a new one. This is a huge improvement from the current bureaucratic layer of HR admin-types which you have zero say in how they operate and which is absolutely not looking out for your interests at all.

It's hard to understand the mindset of "I'd rather just be powerless in the job I have because that seems easier."