Unions appear to give the capabilities for workers to protest poor wages. I've heard positive things from my grandpa about unions when he needed to find work. Everything has negatives but I'm unsure why workers wouldn't want those positives.
[1] https://kk.org/thetechnium/the-shirky-prin/
Unions have a few positives, but, especially in the US, unions have a reputation for historically becoming corrupted by organized crime, strangling businesses into oblivion, and/or making it impossible for companies to fire incompetent/lazy workers. I know it's not like that in some places in Europe, but the US has been burned by truly egregious unions that have set back progress for decades.
In fact, both wages and life expectancy were improving faster before labor laws came into effect.