That’s because developers usually using Unity aren’t aiming that high or simply not putting the time to create one. Then again have you seen UE4 developers come up with Demo scenes which match those of unity or paragon other than Epic and a few large companies?
While their latest demo “Book of the dead” may indeed be using some custom tech to be found in the upcoming 2018 versions, the rest can easily be done in Unity and yes they are not very optimized scenes full of alembic cache files and large geometry counts but they are very doable, especially for good generalist vfx and character artists or animators this is not impossible.