Without a union you're on your own when the shit hits the fan. It might be something "small" like getting bullied at work, it might be some major restructuring that wipes a workplace out of existence, or stops you providing a valuable service to the publioc.
If people don't join their union and make sure it stays under their control it can go bad, because there are always predators out there who are out there to grab any bit of power they can.
But I was a shop steward for years in my union, and I had lots of things I'd far sooner have been doing with my time, like playing music. I never got anything personally out of it, and nor did any of the people I knew who were doing the same - other than knowing that if we didn't do it, it wouldn't get done, and we'd have noone to turn to when things went wrong.
I get tired of people who never do anything to help or get involved in any way, except when they want to get help because something's wrong, and next week they're maybe ignoring a picket line set up because someone else was in trouble or at risk.
What would you think of someone who didn't ever bother to tune their guitar, and then blamed the guitar because it didn't play in tune?