The sky is unnoticed because it is always present and changes only gradually. Sky awareness is low because the sky is no longer a source of important information. Concerning all three, Daniel suggests that the sky was never an important source of information for humans. Experiments have shown that the human eye is significantly less sensitive to stimulation from the upper region of the visual field. This sensitivity pattern has been attributed to evolution.
Humans are upright with eyes on the front of their heads.
Food, enemies, and obstacles are for the most part found in front and below. Thus the human visual system evolved with highest sensitivity in this region of the environment. To see the sky clearly the human must tilt his head back and Look up or lie on his back to place the sky in the most sensitive part of the visual field. My own feeling is that reading should follow the sky experience. I doubt if any people have gained an appreciation of classical music merely by taking a music appreciation course.