i believe part of the issue is that our eyes are our primary source of input. we can control what we see by the direction where we look, or we can close our eyes. we can not control hearing in the same way, and therefore we instead learn to focus or not focus on specific sounds. but that happens much more subconsciously than how we control our eyes, therefore it can happen even if we don't intent to. (ok, when you are deep in thought you can also gaze into nothingness without closing your eyes, but that's less common)