Even if you don't enter Spectre's hyperbolic chamber, these things do add cost for little benefit. Thing is, they are not designed in to lower price models, for good reason. For one, a small hatchback is generally low enough to see the heads of kids out the rear window. Two, they're models least likely to include a nav system or similar. Three, and most importantly, they're built to a cost, so what screens there are are generally not more than two color.
So, the low end of the market would need redesigns, they would need screens installed, and they'd need a markup to justify the added cost. Remember, this is the market segment where the price is the most important factor. For what benefit? I can see out the back of my car fine, the average kid walking down the street is above my window line. Kids should be taught from an early age to get out of the way of a running car - I was, and the vehicles around my area had much bigger blind spots than the ones Spectre and Mitlov are arguing about.
The only vehicles that need them are high, SUV type things - and those don't even need them if your kids aren't morons.