Since there is more than one point resolution anyway.
And also, how is it different for android devices?
Displays are cut from larger pieces of display material at a given PPI. So Apple focused on perfecting a 163PPI display material and then tended to use it on multiple products. They'd do the same thing for the retina display material as well (I think at one point the iPhone and iPad used the same display material and it was just cut to different sizes in making the display)
Apple could do this because they work so closely with their display manufacturers and they care so much about fidelity. It just happened that doing rigid multiples of the original display size was convenient for software as well.
However, displays have become more commoditized, especially retina resolution ones (at the time it was introduced, Apple was the only one shipping these kinds of displays in such volume) and Apple's volume has only increased over the years.
So we're seeing a shift to more commodity display production, I believe, because the commodity market has caught up with Apple's standards. Beyond 300 pixels per inch there isn't much advantage to higher density.
But there is a huge advantage to unit volumes of a display also used in portable TVs or whatever.
The software cost of downscaling isn't significant at this point so it is an economic change.
My best guess is because this would then be smaller than the iphone 6 size (in points) at a larger physical size so everything would look "too big"...
This way, all existing buttons that were designed with 44px tapable height in mind or any existing text could have more details in it while not appear larger or smaller than any other iPhones and will have more space to display content. I believe non-mini iPad is the only product in the iOS family that recommended 44px is physically larger than the rest.
I was referring more to the concept that the entire substrate is built as a huge 163 dpi display and then it's a matter of customer preference how the glass is cut.
(This would also give them back a minor marketing advantage that they had briefly when they introduced Retina, as I'd guess most mainstream Android's will stick at 1080p)