They still maintain full hd because it reduce loads of gpu which can greatly improve of graphic performance and battery saving thus good for gaming. No need for higher resolution if you did not using vr mode because the sceen is small. You cannot differentiate between full hd or 2k display with bare eyes. Thats why apple iphone 7 still using 720p but they improve other features of the display to make it feel good. If you see, you cannot feel the different between 2k or 4k display on android and only 720 in apple. And no i’m not apple fanboy because of the limitations of the os. It sucks!
THANKS