Because with the amount of motion in the camera, there's no parralax on either the fog or the greebly detail in the background.

It never occured to me that the whiteness at the bottom of the painting was meant to be fog. If I was going to redo this, I might try to tone it down. I won't add 3D fog because there's none in subsequent shots down the shaft.

I'm not sure you what you mean about "greebles", but I don't think there are meant to be any lumps and bumps in the wall's surface that would be big enough to show parallax over such a small move.