There is nothing inherent in unions that make them operate like that.
This incentivizes unions to never voluntarily reduce their numbers. The more mediocre the employees, the more protection they need from the union, and thus the more power the union has over the employee population.
The ideal union employee to a union boss, based on how unions are funded and run in the US, is a minimally competent drone that regularly pays dues.
Unions also consistently advocate credentialism over performance. If an employer wants to improve performance the union will fight any outcome based benchmarks, and will instead push for everyone to be educated and certified. In truth it would be best for the business to do both, educate and measure outcomes. What is more important, that every teacher in the US have a masters degree, or that every teacher be good at teaching. The union will always advocate for the former.
I like unions, particularly in the private sector. I wish they operated differently and were more pro-active in cultivating skill (like old school guilds) and less about body count.
Using your same argument, we should abolish all corporations, correct? Since a poorly ran corporation can quite literally kill people due to negligence.
I find it honestly absurd that people attribute all of these negative qualities to unions and yet are not willing to do the exact same thing to the same corporate behavior which has zero checks or balances.
Businesses use lobbyist to get laws to entrench themselves in their own monopolies. If I attempt to disrupt this business by avoiding their regulatory capture, police are sent to punish me. Depending upon which laws I violate, I can be sent to jail and end up with years in prison. Prison, where you are kept in stylized cages and your ever waking moment is controlled by others. That is a treatment that I do not see any significant difference from treatment that would be labeled as slavery.
I know people this has happened to, and I don't see why people treat it as being somehow justified just because the business went through the process of lobbying (and political donations which only seem different than bribery in name only).
And this is ignoring the history of business outright murdering people for protesting for workers' rights.
That union shops may seem to be subject to extra risks of of certain types of nonsense doesn't mean they aren't a win overall.
But yeah, to be honest, some people with unions find they need to struggle with union leadership over some things. It's generally a lot better overall than struggling with your boss.
I'm not sure that there's nothing inherent in unions that makes them promote mediocrity. There could be - the fact that unions add more layers to the bureaucracy and slow things down could very well have the effect of promoting mediocrity.
Generally, if you want people to do the best work, then the people need to be motivated and they need to have as little friction as possible. Unions add friction. In a company where things work, unions would probably add friction and cause more mediocrity.
Note that I'm not saying that unions necessarily cause this, but I am saying that they might be a cause for it. Claiming otherwise with no supporting arguments isn't convincing.
They are violent in the same way Democracies are violent. If you break some arbitrary rule, we put you in a cage. If you resist, you die.