The technical issue is that the center rear view mirror cannot see below the bottom of the rear windshield. And so small children, perhaps on bikes etc, aren't visible in the mirrors.

It it possible to mitigate this by visually checking that blindspot before you get in the car, and keeping aware to make sure nothing sneaks into it while you're getting in? Sure, just like it is also possible to backup with no mirrors whatsoever by getting out of the vehicle and surveying your surroundings.

But on the whole, enough people don't do this that backup accidents are indeed an issue.

What my neck really wants is an auxiliary backup camera that you can stick on a towed trailer.

(Also I'll just chime in here - "screw touchscreens". The damn things should be illegal for any function the driver might need to used while driving. Software defined display screens are fine, but should be fully navigable through straightforwardly-predictable states using directional buttons on the steering wheel)