And yet drivers managed to (mostly) not back into kids for over 100 years, without rear cameras. You walk around the car before you throw it into reverse. The screens fix the "mostly" problem, but they're not a substitute for driving skill.
They reckon the mandatory backup camera law saves 95 lives a year.
Some cars do have remote-control drive features these days, so you can reverse it from outside the car, but that's a super high-end feature (designed for getting in to and out of extremely tight city parking spaces) and relies on the chips that are in shortage, so that's not a solution.