Well it does that too, but that is at most secondary.
UX/UI consistency means both URP and HDRP work with the same logic, their settings can be changed in the same way, i.e. the sky and fog settings can be managed in the same way, the materials of both can use the same textures, i.e. why should HDRP have a mask map but URP a Metallness map with separate Occlusion map? etc etc.
they are working towards a solution to unify HDRP and URP but the eventual unification, if they decide to do it, is years in the future
why there are these difference you have pointed is simple. they had different teams that worked on different render pipelines each with different goals to achieve. Obviously it made no sense to limit the high end HDRP shaders and materials match URP materials. bloating URP to include stuff needed for HDRP is not a good idea also.
why not use a high level interface in the editor that behind scenes would had converted the assets for different platforms without the need to chose URP or HDRP? who knows, maybe it made no sense to do it because the idea was to give you freedom to write your own render pipelines, the URP and HDRP are examples of scriptable render pipelines.
but then they locked these pipelines behind closed source and you can’t get them and modify and build upon if you don’t have enough money to pay for source code access.
so most of us have no idea how to write these pipelines 
even if theoretically modern unity concept is to have custom pipelines.
all those drag and drop files in various slots in ten different places in the editor is a result of the concept of having custom pipelines. instead of a drop down switch URP vs HDRP that would had prepared all the setups internally.
as always with the past 5 years maybe? unity motto is just “adapt and patch and try to keep afloat along the way till things hopefully will get better”