It's not always about lives saved. In the scenario that backup cameras help prevent, the vehicle isn't going very fast. And kids are small. That doesn't mean kids don't get seriously injured as a result. I've known three people that have been backed over, myself included (and that's just the ones I know; I don't survey everyone I meet). Minor injuries for two of them, very-much-not-minor injuries to myself that I deal with decades later.