Because my life is not full of ambient sound. The only noise in my house, other than the sound of cats getting into trouble, is the music or TV I turn on when I actually want to listen to it. My workplace is not entirely silent, given how noise carries through its three-story atrium, but there is no continuous background music. When I am not working with people, or chatting with friends, I live, essentially, in silence. I'm comfortable with that, up to a point.
Silence is conducive to reading, to thinking, to writing.
The sound of silence can be the sound of a mind at work. But it's also the sound of separation, of isolation, of death. Silence is in some ways deeply inhuman. If our species absolutely requires noise, it may be because we need the comfort of our own kind. We need friendship, gossip, news.