Bumper.
Traditionally it was always due to the sensation of speed, these days it's just habit - any other view feels too weird.
It's not actually a cam on the bumper though. I think the camera still resides at the same longitudinal point along the cars length as the cockpit view... it just doesn't display any of the car.
In reality there is a zone in the view that you still have to treat as your car even if you can't see it...
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... this varies by car. Get a long bonnet coupe, and a sambabus and see how close you can get to the car infront before there's contact!