However, using that line of thinking people voluntarily join a company too.
Often to work for a company with union rep one HAS to join, whether one agrees or not, so it’s not entirely voluntary in a strict sense.
All this said, I do believe Amazon is treating their warehouse workers unfairly and they deserve pushback though I admit I do not favor bringing in a Union due to my past member experiences with them.
In contrast, for the union, even if you were forced to join, you'd still have democratic control over the union itself.
So there is a big difference. It's the difference between choosing in which dictatorship to live and living in a (sometimes direct!) democracy.
A choice between starvation and warehouse job at amazon where you’re penalized for taking a bathroom break is not a choice made freely.
They are indeed not a cure, but the cure to the symptoms we are seeing is nothing short of a total overhaul of the capitalist system, which is both not politically feasible, and requires something like unions to make politically feasible to begin with. That is because by definition such a changé even if it was in the larger interests of society wouldn't be in the interests of capital.
Exploitation or at the minimum the potential for it exists in every economic system. It’s not a feature exclusive to capitalism as much as people dream it to be so. Even in a barter economy, can I not take advantage of another worker? Of course I can!
Workers unions in socialist countries were basically illegal, there was only one legally allowed union pretty much. In the Soviet Union the role of labour unions were mostly for the state to resolve interpersonal problems, and in theory to allow the state to receive feedback from employees to optimize production, but not in practice due to dysfunctions because of the broken political and economic framework.
But that brings us right back to my original point: if my choice is starvation or $5/hour, I'm going to take the $5, but that's a coercive choice. I was forced into it by the threat of death. Now explain to me how that's different from holding a gun to someone and forcing them to work? You didn't determine the value of my labor, you determined that I don't want to die.
So an attempt to increase the cost of labor (minimum wages, credentialing/protectionism) is an attempt to extract the true value of labor, even though the threat of death-by-starvation remains. Like a previous commenter said - unless we want to radically re-org our society - that threat of death is irreducible. For example, if every person was granted enough arable land to subsist off of by the state, we'd see a very different labor market.
To repurpose a famous Churchill quote "Unions are the worst solution to labour equality except all the others that have been tried before."