Did you ever read that study about how the brain processes silence differently than music? I stumbled on it a while back, and it basically said quiet actually gives your brain a chance to reset and process background noise without getting overwhelmed. That humming fridge and lawnmower you mentioned, your brain is still taking it in, just in a more relaxed way. It's like the brain gets to do its own housekeeping instead of getting dragged along by a beat.
Actually, I kind of disagree that quiet is something we "fill." For me, the quiet isn't empty at all. It's full of the hum of the fridge, a distant lawnmower, the creak of my own joints on the mat (haha). Tuning into all that stuff, instead of blocking it out with music, is the whole point. It's like my brain gets to relax and just notice things, instead of being told what to feel by a playlist. I notice the same thing in the warehouse sometimes, all the background sounds you tune out until you actually listen.