It's the mentality that if the music doesn't have words, it's not important and thus you can talk over it, read a book over it, think about watering your garden over it, anything but actually really listen to it. A guitarist friend of mine calls it the "AM Radio Syndrome" because he believes it started back in the days of pop AM stations. They would talk over all the instrumental intros to the tunes, right up until the lead vocal comes in and then talk again at any instrumental end of the tune. Subconsciously people begin to think, "Hey... that part of the music must not be important."
I mean, think about it: What was the last instrumental hit? Would any one in their right mind release an instrumental ROCK tune? Or soul/R&B tune like "Hip Hug Her"? The shit just doesn't happen anymore.
Also, we are constantly surrounded by music. In the mall, on a plane, in the grocery store, at the hair-dressers, etc etc. People usually have no idea that music is even playing. I go to dinner with friends who are not musicians, nor necessarily into music as much as I am, and I'll hear something over the restaurant sound system that will make me laugh or want to sing along or whatever and the usual response is, "Oh... I didn't even notice there's music on..." We're so inundated by it, we don't even realize it's there.
It's a true shame, really.