This always reminds me of when I was first learning to use one of the first versions of the Unreal Engine. I made a real big wide open area that had a lot of stuff in it and the game would bog down from having too many things to render, so it was better to have some closed off areas and make walls so the game would not render anything you could not see in that area. So I kept it the same but just added walls or cliffs to block some areas off, So I could still have the same stuff in the level but it would not bog down because it only had to render what was visible.
The universe does not want to waste rendering power to decide what state each particle is in if nobody is looking at it.