The point is, in most cars when a driver is proceeding at a reasonable speed they're looking at the road in front of them. If something is going to end up in front of their car, it has to pass through an area where they're looking. If there is a 5m blind spot directly in front of their vehicle, that only blinds them for the last 100ms before they hit it, so it doesn't really affect the safety of the vehicle.

But, when a car is stopped at a light or something, the driver may be looking away because they're not moving. Same if they're in slow moving traffic. In slow-moving traffic something can enter that zone in front of the car from the sides. In fact, that's one of the most likely scenarios: someone crossing in front of a stopped car, for example. That's when it's essential to have visibility of what's in front of you, which you get with a normal car having a reasonable hood shape, but which you can sort-of get away with using a camera on a i-have-a-small-penis truck.

The camera not operating when the vehicle is moving quickly is fine, because that's a time when the driver should have their eyes on the road via the windshield, and not be looking down at a camera that shows them a 2-d, low resolution view of essentially the same thing.