Wouldn't the entire rear of the car and the rear seats have to be glass to exempt a car from having the camera?

A bigger rear window wouldn't have let me see the toddler who'd wandered a short distance off from his parents in a car park and was standing right behind my car.

Look at the split rear window on the Prius; it does exactly that. If the driver is tall and turning their neck, or short and using the center mirror, the sight-line from the driver's seat extends below the rear seats and down into what would ordinarily be the tailgate.

You can't see the pavement under the bumper, but you can see a kid on a tricycle right behind the car.

Personally I find rear view cameras to be quite disorienting. The view is quite distorted by the wide-angle lens and it's not clear where it can see and where it can't. Do I still have blind spots if I look at only the screen? Where are they? With a mirror I get an intuitive sense of what it can and can't see. Also, I expect the view to move when I move my head like in a mirror, which is part of what makes it so easy to get a sense of where the mirror can see. I suppose someday I'll get used to cameras, but today is not that day.

Do they still sell any cars with good rear visibility?

My hatchback has about the best you could get (and they don't sell it here anymore) and I still added a backup camera, it's very useful.

I used to pride myself on my ability to fit into small parking spaces and parallel park even big vans pretty precisely, and a backup camera blew all that out of the water because I can fit anywhere now.

My little GTI with a backup camera made me an invincible parking machine.

Why? You have a screen, the cameras are so small as to be undetectable, they add safety and they provide much better coverage than mirrors do.

It also enables auto-parking and other enhancements while not inhibiting anyone that wants to perform those functions manually.