Use your brain, Dan. This is a requirement of the technology. It's because that is the dot pitch of a 5.5" diagonal 1080P screen that fits on an iPhone 6 Plus, 6S Plus, and 7 Plus. It also happens to be the video HD specification for high definition video movies that would be video recorded by the camera included with the iPhone 6 Plus, 6S Plus, and 7 Plus, and then shown on 95% of the big screen television sets that are owned by the general public which are ALSO 1080P. There is no other reason than that.
To use a 326 PPI dot pitch the iPhone 6 Plus, 6S Plus, and 7 Plus screens would have had to have had 6.76" diagonal screens and been far too large for Apple's specifications. (Do you recall the early 1080P Android phablets that came out in that exact size?) Otherwise, the iPhone Plus models could not have had the HD proportioned screens.
Apple did not increase the PPI because it was necessary for improving visual acuity or to improve human perception beyond the Retina ability of the 326PPI, but rather to maintain ease of HD proportionality in a 5.5" aspect ratio using an efficient one-to-one video camera ratio to video display screen ratio without having to do inefficient algorithmic transformations (something every Android phones with non-standard pixel size screens must do, which again slows down the display) to fit one to the other. It's as simple as that.
I will wait for the iPhone 8 that will bring back the headphone jack.
A ha! So there ARE reasons other than “you can’t see it!” to go to higher resolution. And with UHD and 4K video becoming more popular, you want higher resolutions than 1080x1920 to support native resolution.
Funny, it’s about time that Apple understood that native resolution modes are big benefit! Most other vendors got that several years ago... Kudos to Apple for finally catching up!