Well the reason for that is because of the resolution which 5 and 5S had. To sort of scale it instead of coming up with an entirely new resolution which would've created more work for devs. 2x assets. Same thing really goes for the 6 Plus which actually down samples 2208×1242, 3x assets, into 1080p.
Seems like quite an eccentric way to support higher resolutions? I realise at first things need to be upscaled. But if Apple implement a single new resolution that would like be around for a long time (1080p) then developers would quickly start optimising their apps for such a resolution.