Here's the thing: watching a monitor doesn't actually update one's spacial model of their surroundings and their vehicle's position therein. Under normal operat

Here's the thing: watching a monitor doesn't actually update one's spacial model of their surroundings and their vehicle's position therein. Under normal operating conditions an attentive driver has and is constantly updating a fairly accurate mental model of their vehicle and the objects near it that they are likely to interact with. When your attention is drawn to a rear camera system you've now ditched that spacial model entirely and have changed to modeling the video game on the screen in front of you. This context switch is -very- expensive to unwind and at least momentarily costs you your awareness of where everything else except the back bumper is in space, and what's in front or to either side of the vehicle. As far as mirrors go, they aren't to be relied on solely as they do not provide a full picture of the vehicle's surrounds. Checking one's blind spots by turning your head and actually -looking- in your blind spot before merging is a thing. same goes with the rearview. Hope that clears up your confusion.