They aren’t here against their will. They are here working for the trafficker against their will.
Basically the trafficker makes a deal with them to pay to smuggle them in, in return they have to work for them for a certain number of years to pay it back (kind of live indentured servitude agreements).
Usually they end up working more hours and in worse conditions than the original agreement, and get room and board deducted at higher prices from the debt and other forms of getting taken advantage of, but because they are illegal and don’t speak English, they don’t have the ability to complain.

Lau