Whether supposed purists like it or not, so much of the modern car is digital. The throttle and more recently the steering are by wire now, there's a big screen in most cars with all the Infotainment gubbins loaded onto it, and some come with creature comforts like rain-sensing wipers, dark-sensing headlights, automatic climate control and radar cruise control that slows down for you if you're too busy tweeting #YOLO to your other idiotic friends (but hey, if someone needs to be told that "You Obviously Love Owls", then telling them should be priority one, right? Right?). Many cars even have reversing cameras with a superimposed parking aid display on that there TFT screen, which leads me back to the titular question of this here post right here: why not cameras for all the rear views?
Concept cars, like the Porsche 918 Spyder posted above, have had little fins or sticks with cameras on them for a few years now, or in the case of the surprisingly handsome Panamera Sport Turismo Concept, integrated into the side vent for the smoothest airflow. Airflow is an important part of modern cars of course, because a slippery body means a low drag Cd, and it also gives a smoother look to have a wee camera on the door instead of a big chunky mirror. Light sensors would give them the auto-dimming qualities of modern glass units, and mirror-sized screens mounted at the ends of the dashboard top would be right where your eyes naturally look for mirrors, so it wouldn't feel that different to use, but would add an MPG or two (Fun Fact: when the DeltaWing added glass mirrors to replace the cameras by request from the ACO, it added 8% to the car's drag coefficient because it was so low-drag to start with), look cooler and, as this rough picture shows below, would be within the width of the car, which goes some way towards negating the counter argument that they would be expensive to fix when you clipped or were clipped by another car and it broke off, because the chances of that are significantly lowered:
But I know what you're thinking: Is it safe? Technology is great only when it works, after all. What if you started the car and they didn't switch on or had a faulty connection? I'll admit that's a potential flaw with any early attempts that don't age well, so perhaps the stalk with the camera on it could have fittings for backup glass mirrors until the problem is developed out (the tip could pop off to reveal a tripod-style screw thread, assuming the camera in the above image is mounted a tad down from the top of the fin). The other potential issue is the picture they provide not being clear enough for high speeds, but again this would be something that would improve quite quickly over time. Considering the quality of some cameraphones and compact digicams, it probably wouldn't take long for the picture to be HD, although I draw the line at 3D door cameras, because 3DTVs are a pointless fad. That said, 3D parking cameras wouldn't be the worst idea...
Really though, the most beneficial place for digital rear view is indoors, above your head (not directly, mind). What with being part of a family and all, I've spent a lot of time sitting in family cars, and with a family hauler full of humans, camping gear, cardboard to take to the tip to recycle or all of the above, it's not always the clearest view out when you look at the central mirror, especially if someone's sitting in the middle. Enter an HD camera mounted at the top of the rear window. No interruptions from front to rear, just a clear view out, perhaps with distance readings when reversing, and auto dimming for that jackass with their high beams on. Oh, also night vision. Almost forgot about that. When such cameras become commonplace, an ability to actually see into the pitch black night behind you might well be beneficial. If the interior screen system fails, switch the screen off and rotate the unit to reveal a glass backup mirror mounted on the other side. Just in case.
A rearview camera system for the interior mirror is already used in the Gumpert Apollo, what with that not even having a rear window and all, so it wouldn't be a big leap to imagine it in a normal car either. Initially all this would be notably more expensive than the traditional option expensive, yes, but so was HD TV once upon a time. And flatscreen TVs. And TVs of any sort. And cruise control and all the other fancy stuff in modern cars. And the car itself. When something becomes commonplace, it develops and gets cheaper. You can't deny that in the comments via your pocket touchscreen camera-equipped telephone...
So what's stopping it? Regulations (and should they be changed)? People playing it safe? A flaw I haven't thought of?