So lets take an honest look at modern slavery.
Despite the dire conditions of the developing world in terms of sweat shops and the literal lack of value to the individual human life, I do not consider those a form of slavery rather than an oppression of labor.
The simple fact if it is, is that if you didn't want the exploitative systems of labor to exist in these developing countries, then it would require hard decision which I'm not sure the average first world citizens would be prepared to make, even though in the long term it would benefit them. (Support of local industries, local jobs without the leftists or wing nut unions involvements, and being willing to pay a higher price for that T-shirt.)
That is the eradication of cheap industrial goods that are produced by that labor. There is a reason that sweat shops exist. They fuel the local economy and allow the interchange of wealth from the developed world to the undeveloped world. Unfortunately the person on the very bottom wrung of this ladder, does not see a vast amount of that wealth redistribution, and consider how many 3rd world bosses are the very epitome of avarice, they are not likely going to see grand improvements, but now they are in the system there is no place for them outside it.
They could return to subsistence farming. (Guaranteed poverty without knowledge of better farming practices.) but a lot of them have been removed from the land now for so long, that returning them to agrarian practice wouldn't work. (Cambodia under Pol Pot tried this. It was massively unsuccessful, as the mountains of human skull will attest to.)
In much the same way the industrial revolution in the west took subsistence farmers and brought them into the factories and generally out of poverty. (Despite the appalling conditions, a lot of families were able to make it.) And lead to the development of prosperity, land, and wealth that the west had only managed to destroy through two costly world wars, and several different political upsets. (The irony being, that the WWI & part of II were the creation of factory output never before seen in the scale of private industry, and actually empowered the working class enough that they created the back bone for the labor movement.)
That said it took a lot of time, ingenuity and human misery to create the situation of industrial wealth and prosperity that had reached its peak in the late 60's from there, a lot of the actual industrialized world changed from a mass exporter to mass importer and service industries and finances became the key economic sectors not industrial output, which is for the most part turned over to the developing world.
Now with the out of the way, where do many instances of modern slavery exist?
Well mainly in Africa and the Arab world in open. While it is illegal to own slaves in Saud, these so called "Servants" that exist are basically legal slaves in this regard. Amongst western countries the greatest cases of modern slavery occur in migrant populations. (Indians and Pakistani's are especially self predatory, as well as the Chinese.) Or in the sex trafficking trade, which ironically is mainly a white on white endeavor and is Eastern Europeans being used as brothel labor in the West. (Way to go Schengen zone.)
None of these contribute to the economy in a meaningful way. The guy who is working in a cheap labor job and living ten to a house is often either illegal or does not have right to abode, and therefore is not able to work in a legal capacity anyways, but instead works for this modern slave owner who is usually a landlord as well as employer.
They cover their illegal labor, by not only paying them under the market rates for labor, but as well crowding them into what are essentially flop houses and charging them rental rates which is usually taken out of their pay, so their already paltry fee is essentially reduced.
What money they do have, they usually send home, though even in some of these cases the boss acts as a banker, because the employee can't get a bank account, so again it's rife for abuse.
Aside from sales taxes on the minimal amount of money, this person will be spending, they're essentially a vampire on the economy, because they are using the infrastructures in place, without paying any meaningful form or tax, and neither is the employer, or not in a way that isn't being fudged, laundered and used to fund sweatshops back home.
And it doesn't matter if the employees get seized, because with loose border controls, and a toothless police service there is no way for there to ever be a run off of cheap desperate labor that will be predated upon by their countrymen in the guise of doing them a moral good.
Same rules apply for North America though it's often Mexicans or Somali's or other imports that can get traded out.
And why do people allow it to happen? I think it's partially because it's not actually brought to the fore of everyone's attentions and why would it? The media is sympathetic towards the idea of globalism and economic migrants, and so unless it's a truly tantalizing story they aren't going to write something that contradicts their agreed narratives.
At the same time anyone who tries to point out the fact that they undervalue the market is either labelled as xenophobic or a racist, or anti-capitalist, despite the legitimate business being at a disadvantage because it's following the laws that the country had created, rather than just doing it's own thing.
Finally the major disappointment I find is in the lack of common sense thinking on behalf of the consumers/citizens of the products produced.
Whether that is cheap clothing, cheap labor, cheap food. Everyone is primed to obsess about some perceived form of deal, versus real world value. And I understand that the counter argument to this is that people need these cheap consumer goods in order to be able to function in these high cost high living standards countries they live in, but it's a lie.
People devalue their labor markets, flooding the lower and middle tier ends with cheap easily replaceable labor then complain about low and stagnant wages. In places where the industries are already moving towards an automation/technician style labor force.
They complain about the failure of native industries, yet buy bulk cheap goods from other countries because of a price differential. Despite the fact that the more expensive native brands will last longer and perform better over a longer period of time.
They complain about affordable housing, social program costs, and failing infrastructure, while allowing their governments, left wing organisations, and major business players to push through waves of hundreds of thousands of people who will become a strain on these services and compete and in most cases outbid the same group of poor people that the programs were originally created to protect.
They push their children to get semi-useless degrees in colleges and amass large amounts of debt in the process which the degree itself won't likely pay for, while ceding traditional industries to the new immigrant classes who end up becoming so embedded in these industries that they in time come to dominate them and can further push out any native labor.
