Why does slavery exist today?
Slavery exists today largely for economic reasons. That is, there are enough places where people are so poor that they will take risks that end up causing them to become enslaved. Of course, this could not occur without the presence of people who are unscrupulous enough to take advantage of people's poverty.
Much of slavery today comes about because people in poor countries will take risks to find good jobs. They will, for example, believe recruiters who come to their home towns offering good jobs in foreign countries. They have so few prospects at home that these promises will seem attractive. What often happens is that these recruiters are actually taking them to be enslaved in various low-skilled jobs. They can be kept in slavery because they lack the resources and the knowledge to enlist government help for themselves (and because in some countries there is little chance the governments would help them anyway).
Thus, slavery today is mostly caused by poverty rather than by raiders taking people and forcing them into slavery as was done in the time when Africans were being brought as slaves to the Americas.