Human beings like to talk – some talk quite a bit – some hardly seem to stop – and for all that, may rarely touch on the largely unspoken aspects of our experience that we tend to take for granted. People can talk a lot without addressing anything writers note in setting a scene, or mood, depicting action, evoking memories and dreams.
Most human speech would come under the heading of dialog – and this is why those who may talk up a storm don't necessarily write great reams of fiction. You may get talkers who also write prolifically, but the two don't necessarily go hand in hand.
In writing fiction, we take a step away from the inner monologue to notice and listen to a world of things that may ordinarily go unmentioned. Who comments on the colors of asphalt and dead grass or narrates their own activities so far as to note each nod or half-smile, each tucking of hair behind an ear or adjustment of position? Who analyses stray thoughts and fleeting feelings as if they really mattered? Who talks about the mood and lighting of the places they happen to visit?
Who identifies so strongly with people who may live only in their own imagination as to put the thoughts of those imagined people into words?
Authors. Maybe other people do that too, not all of them considered well-adjusted, but this all seems to be business as usual for writers of fiction.
It seems to me one of the great virtues of the art. It's too easy to think that things left unspoken are therefore unimportant. Writers correct that misconception in every work they perform.