This is like saying if you don't like the gangs in your area you should join them and try to reform them to do social work instead.
Why isn't it valid to say you don't like unions and so don't want anything to do with them?
Because you can't possibly have personally experienced every union, and the media isn't good at covering unions generally, so whatever bias you're (or anyone else making this statement is) presenting here feels unearned.
This is similar to the bias of racism or homophobia, right? Like, "I saw a Black person steal something, so now I think that all Black people are untrustworthy." Or, "I saw a gay man hit on a straight guy and now I think all gay men are sex pests." These are declarative statements about all people in those groups based on a small amount of information and on societal bias.
And these biases rarely go the other way – do you also end up saying, "I don't like corporations and don't want anything to do with them?" Corporations are, by far, have a vastly worse record on theft, corruption, anti-environmental practices, etc.
And nobody has to earn the right to decline to be pulled into a political organisation they don't want anything to do with. If someone doesn't want to join your club the right response is to think about how to attract them, not to ask them to justify why they don't want to join. Otherwise do you join every single political party? They all claim to represent your interests!
I'm not realistically able to live my life without working with a corporation. I am demonstrably able to live my life without a union. So the bar for the union entering my life, adding that extra complexity, taking some of my money and borrowing the little power I have, is of course far higher.
It's right to be extremely skeptical of anyone who claims to want to represent your interests. Because usually they really want to use your power to represent their own interests.
Because they are democratic institutions. They can be held to a no-problems standard because every time a problem occurs then those involved can be sacked and replaced. The level of acceptable corruption is set by the membership. Rules should therefore focus on openness and member participation in leadership selection. Everyone who hates unions because of X or Y, to them I say join the leadership and run the union the way you think it should be run. Tearing down the system doesn't fix anything.