A lot of people are using fake ID's. Fingerprints would be under a false name used. People from f.i. Honduras, Guatemala, mostly travel through Mexico and a lot of them have a fake Mexican voters-card for the reason that if they are send back they are send back to Mexico and not their country of origin.
As an example, this is about how they struggle in Arizona to identify the deceased border crossers/migrants.
Arizona: Naming the dead from the desert
In this case there is a pretty recognizable picture of the deceased. It is even possible his family knows already he passed, but there are communication or money problems, family has to travel to the USA to make a formal identification and have no money to do that, let alone repatriation. It happens a lot.