Here's why, and it's obvious once someone says it out loud: marker is ink that sits on top of the paper. The printed address is ink that soaked into the paper. Put light behind it and you see both. The marker was never covering anything. It just made it look covered.
Like taping a Post-it over a window and calling it a blackout blind.
I tested five different markers on a fresh label to be sure β Sharpie, a chisel tip, two cheap ones from a drawer, a paint pen. Same result every time.
"But mine are stuck to a box, not held up to a window."
Mine were too. That's less protection than it sounds like.
Cardboard gets flattened, torn and crushed β in your bin, in the truck, at the sorting facility. Labels lift off on their own once the box gets damp, and plenty of people peel them off deliberately before recycling. The moment a label separates from the box, it's a loose piece of paper again.
And you don't need a window. A phone torch held behind it does the same job. So does tilting it under a lamp.
Or skip the light entirely and just scratch at it with a thumbnail. Marker sits on the surface β on the glossy labels couriers use now, it comes away almost immediately. I'll come back to that in point 6, because it turns out most of the products sold to fix this have the same problem.
If you want to check for yourself it takes about four seconds. But you can probably already see it in the photo above.