Consider also that in the countries which still allow sweat shops, information about a "better life" is not as available as it is elsewhere. I agree with the ideas in post #2 and add that the workers themselves likely have no idea that life/conditions could or should be much better. It has been said that knowledge is the first step to success in fighting for improvement, in anything.
Not only is there a general lack of knowledge about better conditions and how to get them, but there is a lack of people advocating for this on behalf of such places and groups.
Workers have tolerated sweatshop working conditions in pretty much every country that has ever industrialized. Workers in some industrializing countries right now are tolerating these conditions. Workers today and in the past have tolerated these conditions because that is the best work that they can get at that time in that country.
Sweatshops spring up in industries where the workers are unskilled. In this sort of industry, the only thing that matters is cost. The factory that can turn the product out at the lowest cost will be able to get contracts and market share. Because of this, the factory owners will try to get as much work out of their workers and will try to spend the lowest possible amount on providing a safe and comfortable working environment.
The sweatshop workers have to tolerate these conditions because they are not able to find better jobs. They may not have the training or education to qualify for better jobs. Or such jobs may simply not exist in their country. Either way, the workers are not able to find better jobs and so they must accept the sweatshop working conditions.