Item descriptions are the most efficient way to deliver small, optional, exposition dumps on the player. At some point, a story teller has to explain their world some how, you cant realistically communicate every single detail purely through diegetic means with out pissing off and boring your audience to death. If every piece of world building has to emerge naturally through dialogue or environmental story telling, it would be boring as fuck. So you either just hide that lore from the player or you never bother to begin with, Settings feel more immersive when they don't feel like they are constructed for the plot that's currently happening, an entirely diegetic fueled story would have to cut out all the little things that make a world interesting.
