Probably for Artistric value to match what they have been doing with the loading screens and also maybe so they could get it onto last gen consoles without having to totaly redraw all of the textures. there is usualy more than one reason for the choices developers make.

games texture sizes have very rarely matched the screen resolution, for example last gen mostly used 256x256 or 512x512 textures, this gen is mostly 512x512 or a little higher.