Despite having so much technological advancement, I wonder why cars still uses mirrors (right and left side of the driver) for the rear view. These mirrors can be easily replaced by cameras and monitors (display for the driver). I noticed such technology exists but it is restricted to only when you take the car in the reverse direction. If we use this technology while driving the car (i.e. in forward direction), I believe it will be helpful for the driver since multiple cameras installed on the rear side of the car will give consolidated view to the driver. Also it will save the hassle of monitoring both the mirrors while driving. I think this will have two benefits:


Since the mirrors will be replaced by cameras, it will reduce the wind resistance for car.
Weight of the car will be reduced negligibly assuming electronic circuits will be lighter than mirrors.

So, my questions are


Why cameras (and obviously displays) are not used instead of mirrors? 
Does such technology exist? If it exists then why it is not widely used?



This is another case of fancy new (SHINY!!!) technology being A Bad Idea(TM) .  When you're driving, your eyes are focussed roughly at infinity, i.e. looking at objects more than 5 m away.  When you look at a rear view mirror, you are still looking at distant objects.

But when you look at a camera display, you're focussing on the image screen, which is probably 30 cm away.  This means you need to constantly change focus to be able to look ahead and check the rear view monitor.  Not only is this physically exhausting for your eyes, it leads to a significant time period when you're not focussed on either area.  

Cameras for backing up, which (I hope) happens only at very slow speeds, may be helpful.   Blind spot coverage is generally an assistive system which does not provide direct view (just warning lights or sounds). 
