I mean people have tried it and there is no law that forces people to be in a union. But why would anyone go and work for a company that is paying them less?

Economic downturn where there are barely any jobs, except at companies that can hire people at lower rates? Rural area and you don’t want to move for a job, but the only companies around pay less?

It’s why I prefer a nationally mandated minimum wage, just not US style where it’s been left the same for decades. Here in Estonia, it’s updated every year and is the product of negotiations between an organization that represents unions and another that represents employers. So kinda like in Denmark, I guess… Except the number they come up with is mandatory for everyone in the country, whether or not you’re in a union. Obviously a union will still have its own collective agreements and can demand higher wages, including by striking if necessary.