Because there are plenty of times they make things worse?
Let's assume the absolute best of intent, no corruption, etc.
There is some good, of course. I won't deny this. I'm not going to talk about the upside, but please don't whataboutism me simply because i'm going to talk about the downsides - you asked whether it is irrational. It is no more irrational than any other moral/ethical/etc viewpoint that someone chooses over a financial one. I am plenty aware of the history and good they have done, and the horrible labor practices they helped put an end to.
I just do not happen to be an ends justify means kind of person.
their are a few countries where union equivalents have an explicit goal/requirements/etc around societal good - i have no issues there, the rest of this is about the ones that don't.
For unions, overall, their goal is not to help society, it is to protect and help their members. They have no duty to society whatsoever. In the extreme, that ends up bad for anyone not their members.
That is, they do everything they can only to get their members more money, more benefits, protect their jobs, etc.
This sort of thing comes at a cost too.
So they make it harder to fire bad cops, bad teachers, etc.
When a municipality or state would be bankrupted by paying pensions, the unions will force them into bankruptcy at a cost to everyone else.
The unions will first force them to raise pensions, too, to unpayable levels. The state/municipality has essentially no practical power against its workforce in that situation, so it agrees, again, at a cost to everyone else.
The unions of course, do not care about the consequences for anyone else. They want it paid anyway.
I have no issue with pushing for a living wage, etc. But again, it's for their members, not for everyone, except where they believe it will generate more union membership.
So sum total, they do not care about anyone else. Literally. It's not their job.
This is awesome for their members. Horrible for everyone else.
So no, it's not irrational. Not everyone wants to group themselves with organizations designed to literally not care about anyone else.
Follow the US-style unions to their logical extreme - everyone in society in is a union, divided by job. Yay!?
I think that ends very badly for society - you really think the unions won't paint other jobs/unions/etc as the enemy?
They do in plenty of cases already!
So - i think it will end up the same sort of strong polarization/hate that exists across political affiliation, but across jobs instead. Great.