Because most GPS locators these days supplement their data with triangulation in other ways besides satellite.

Your rentail car is also not a moving platform the way a plane is. The accuracy of GPS will be reduced by speed.

As mentioned, locators report their location via radio/celllular signals. The AF flight's transponder/radio went down.

How exactly would GPS (which requires electrical power) or tracking (which requires the plane communicate its position via electrically powered radio) have made a difference?

In the event of an electrical fault which led to the loss of power to radio, none of those systems will work. You can't make the assumption that the point at which the last known position of the plane is automatically the point where it plummeted out of the sky straight down. So it still comes down to search-and-rescue.

The plan crashed in the middle of the ocean. GPS wouldn't have changed the outcome in any way, and not having a blackbox inside the plane would only have reduced the amount of information available to investigators.