In my opinion is about creating the Lore, myths and legends. NPCs will talk about items that no one knows how to get, etc..
That gives a RPG atmosphere to the game, players will wonder about it, will try to find out, is a mystery, it motivates people.
If everything would be obtainable, then part of this whole RPG background would be lost, also contest would lose a bit of value, since some unobtainable items are only available on contests, like Yellow rose.
I like that we have that, I've lost some hours back in 2004 trying to reach the Crown room in MOLS, Carlin.