Here is why it is not feasible to use a 1080p screen.
Adopting the new form factor, the iPhone XR and the iPhone XS Max were given the same base coordinates of all drawings which are specified in points. Points are abstract units that only make sense in a mathematical coordinated space. This is the reason why the XR is able to have the same amount of information displayed as the XS Max. This special unit(points) is expressed as 414 by 896. The only difference is that it is rendered at 2 x points on the XR and 3 x points on the XS max. To have a 1080p screen the XR would have to render at 3 x points, after which it has to down sample the results from 1242p to 1080p. This increase of work load means the iPhone XR will require more processor power, more ram, more energy while at the same time generating more heat for a small difference in resolution which most people would not be able to visibly perceive.
To express it in numbers on a certain “on screen” benchmark, while the iPhone XR is able to bench 58 fps the XS Max can do only 47 fps. so the difference is exactly 11 fps more, or in percentage 23%. Not taking the difference of screen technology and ram allocation into consideration we can say that the iPhone XR has a 23% headroom over the XS Max. Now this is not even considering the workload of down sampling to 1080p and all the problem that brings. Can you imagine the amount of energy saved with that kind of disparity between the 2 screen just base on resolution assuming the same content at the same fps. Which is why the iPhone XR is the current King of battery life compared to every other iPhone.
In short 1080p cost too much resources wise, yet results in no gain(perceivable visual difference) for most people.
Below, is the above said benchmark.