The hardness scale isn't what determines how quickly a knife blade dulls. Hell, the quickest way to need a new edge on a knife is to cut paper with it.
A knife blade isn't the nice coherent surface you see, its made of thousands, probably million of these little tiny teeth or burrs. When they're all straight your knife is sharp, as it gets used they start spreading, going in opposite ways and being bent back over themselves. Thats what gives you a dull knife. A sharpening steel or oiled leather doesn't carve a new blade, it properly aligns those little teeth again.