I can only surmise that everyone from Hollywood to the BBC to amateur makers of YouTube videos adds "background" (or sometimes more like foreground) music because everyone else does. Some of them even add percussion to news bulletins. It drives me up the wall. The worst instance I've ever encountered was music added to a Royal Institution lecture. I emailed them to ask what they would think of someone present in the lecture theatre playing music. They seemed not to see the point.