Since you're barely awake, and your son/daughter is indeed away (eg. on night shift or staying with friends), and the fact that some cops are corrupt, and somehow you can't call (busy line) your son/daughter to verify, make people fall prey to this scam.
The interesting part is: how do they know the relationship of the victims, and the location (i.e away from home) at that point in time, considering the scammers are sometimes from different island?
Probably they don't. If the victim says "but I don't have a son" they just hang-up. Scattershot approach, not targeted.