I agree that not all unions are fantastic, but staying on the sidelines when you could use your position to influence for the betterment of everyone is a very privileged, selfish act.
I think voting mostly works in democracies by being able to vote a party out after the government has abused us against our wishes and their promises.
In a first-past-the-post system, you often get to chose between two evils... Imagine you were forced to pick a partner for 3 years, one which you suspect will rape you and the other you think will beat you, and you must pick one or the other...
And that is ignoring the fact that usually nearly 50% of the people that voted, voted against the party they got. It is ridiculous to think that the ~50% that voted against a party deserve what they get.
That said, I like this version by H.L. Mencken better: "People deserve the government they get, and they deserve to get it good and hard."
I have seen enough actions by unions where the outcomes were detrimental to their members for me to believe that some unions are a net negative for their members.
Someone can choose to not belong to a union because they are a good person that just believes that a union is bad for everyone.
Edit: Disclaimer: I am not stating that I think unions are bad. However the few direct experiences I personally have had with unions have not been a clear positive for the members (as an employee or student, never as an employer).
Edit 2: Or you could just believe unions are net neutral, so be against joining. Because who wants to voluntarily submit themselves to an organisation or people they don’t believe in?