The catch-22 is that these people are nearly always immigrants, and the criminals have taken their documentation, so the best case scenario is they get rescued and then deported, possibly via a spell in immigration detention. The worst case scenario is the cops turn up, laugh, collect the day's bribe money and then the person who called the cops gets beaten.

(this is an important dynamic in sex trafficking as well)

The government is somewhat complicit - there are even reports that the police take escapees back to their captives for a bribe