Your ability to also tell that voice "but of course that's ridiculous for a large number of reasons" is the governor that prevents the voice from being a problem in most people who do have an inner monologue. When someone thinks their inner monologue is something else's voice, like a diety, that's when it becomes a problem because it removes personal agency while elevating the significance or validity of their thoughts.

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.

An internal monologue is a normative experience. We really do live like this, for better or worse.

it's not an outside voice telling you to do something, it's just forming thoughts as word to go alongside images and emotions. If you do a lot of reading and writing you will start to do it more. Everybody does it a bit, and everybody also sometimes thinks intuitively and quickly off of images and emotions only. The whole idea of two "categories" of people is almost always bunk (introvert-extrovert psuedoscience for example, there aren't 2 species of humans who 'gain energy' in opposite ways. There's just people who have a more developed set of social 'muscles' where they are not as overwhelmed by social interaction)