Unions can be a deterrent to this. If you have sectoral bargaining, you can’t get a better deal anywhere because everyone pays the same. Or even if it’s within a single company, you’re just stuck with whatever scheme the union bosses agreed to.
But also, unions are a deterrent to new employers since fewer people want to start companies in industries where unions have taken over. They’re a nightmare to deal with. Fewer employers means less competition means worse services.
We have a dynamic economy with lots of competition. It’s easy to switch jobs and find a better place to work. We should not want to give this up.
I don’t have any objection to voluntary unions. Meaning people can join them for support or collective bargaining, but that citizens can just opt to not employ anyone in the union. Unions could be support structures for workers, instead they are mafia bosses.