That's the whole point. People do not voluntarily work for shitty wages at shitty jobs, they are forced by the need of money.
Workers in western countries are extremely lucky, since they live in very wealthy countries where even the shittiest job usually gives the possibility to live a decent life.
However, I'm not sure (to make an example which is extreme in the opposite way) congolese children do voluntarily work in cobalt mines, getting intoxicated and often dying just to be able not to starve in the short term.
Regarding the rest of your comment, I just have to say that:
1. yes, people can start their own ethical corporation, and some people actually did that. But they are pretty much irrilevant in the global market. Actually, I would argue that following ethics often puts a corporation in a less competitive position.
2. just because I could do that myself doesn't mean that's a good thing. I'm not saying that the capitalist class is evil and trying to destroy the world, I'm just saying that the system is flawed in allowing such a restricted pool of people to accumulate so much wealth, and to actually encourage the exploitation of people and resources in order to stay competitive.