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.