I have no outside basis to believe it, but I believe this anyway: It's a learned behavior. I don't think it's something you should worry about, plenty of people don't. You still have the same interiority as everyone else. Just never fell into that pattern of behavior.

Meditation often talks about "quieting" the inner monologue. I've tried sporadically (whenever I actually remember it, really) and for me it feels... like un-clenching a muscle that you've held clenched all your life. It's at once relaxing and difficult, because that "muscle" will just clench back up - the monologue starts rambling - at the first moment I stop actively trying to "quiet" it. I get the sense that if I stuck to it and was stubborn, I'd eventually be able to quiet it without actively trying.