The assumption is that if people are alive, they will take the PLBs/ELTs out of the plane with them and turn them on. The case for an auto-jettison ELT is for finding the plane when no one survives and for when the landing is bad enough that some may survive with no chance to get the beacons. Routine in flight tracking is a solved problem with equipemt now being rolled out, as has been mentioned in this thread a few times if you scroll up.