Ask HN: Why don't phones have stereo cameras?

I was doing 3D stereoscopic and augmented reality research in college almost 20 years ago. The cameras, video cards, workstations, etc were pricey, BUT some pretty neat results could be had.

Fast forward to 2017, we have powerful computers in our pockets with awesome sensors. We also have seen some push into the virtual reality market using the phone as a screen.

My question: why haven't the phone makers (specifically Samsung) started making phones with two cameras on the back face, spaced at an approximate distance of the human eyes?

The size of the phone has seemed to have stabilized where the length of the phone is approximately the width of the average human face (a result of intelligent design perhaps?).

The precise width separation of the cameras doesn't have to be exactly the width of our eyes to give the illusion of depth.

It seems that the ability to take, send, and view 3D pictures and videos is ripe and relatively untapped. Also the post-processing opportunities (depth-of-field, etc) seem much greater with stereoscopic images.

Any thoughts?

(My prediction is that before 2020, stereoscopic cameras on smartphones will become commonplace and perhaps so obvious that we will wonder why we didn't do that sooner)