[ohnope]

I'd guess it's because GPS / cell phone tracking data rely on line of sight to towers / satellites, but GACT works on human-to-human proximity, and therefore data can survive offline locally until some point in the future when you get connection back. Also it seems the distance accuracy between human to human would be more accurate (via BT signal strength) than distance accuracy in a GPS / cell phone tower situation.

[Mindwipe]

BT interference is much more common, and BT identifiers are much easier to reset.

Cell phone tracking data does not rely on line of sight to towers, and for most people there is no offline for this to be relevant.