That's why you check all around the car before setting off.
Yes, in principle one could take whatever other measures necessary to prevent such accidents. In reality, backup cameras save lives. Just like seatbelts, anti-lock brakes, crash safety standards, and other safety features that "Real Manly Drivers" protested against back in the day.
So yes everyone is right, yes a lots of people are just bad at taking basic safety measures but backup cameras are still a necessity because this will not change, it is even worse with people doomscrolling their smartphone while driving.
How does that help other people walking down a sidewalk?