Also you generally know who is in a car.

If its my car, and i'm missing. Its my body. If its my car, and i'm at home, its my partner driving my car, or i knew who borrowed it.

Cars are tagged with a plate, and VIN and other details that make it easy to identify. Less people means easier to find wallets/phones/identifiers in a clean up action.

Also the extent to which the pieces of the puzzle may get scattered, I guess. A car crash rarely leads to body parts strewn across entire fields.