Simply put: Your brain has learned to ignore it completely, or nearly completely.
The simplist and probably inaccurate way I can show this: Look to your left and right with both eyes. You might see something of your nose, though it mostly appears invisible. Now close one eye and look in the direction of your nose. You can see it, as you would expect. So it’s there and quite in the way.
Simply put: Your brain has learned to ignore it completely, or nearly completely.
The simplist and probably inaccurate way I can show this: Look to your left and right with both eyes. You might see something of your nose, though it mostly appears invisible. Now close one eye and look in the direction of your nose. You can see it, as you would expect. So it’s there and quite in the way.
If I just look left and right without also looking down, I still don’t see my nose. I see the bridge; but not the rest of it.