Anything out on a (well traveled) sidewalk is going to be constantly moved around by traffic (and weather) causing it to abrade against the sidewalk itself (and

Anything out on a (well traveled) sidewalk is going to be constantly moved around by traffic (and weather) causing it to abrade against the sidewalk itself (and whatever else is around) causing it to 'weather' away (round off). Pretty much anything old we find like those knives are pretty much guaranteed to have been essentially buried all at once, and then left more or less undisturbed from that moment on. Minimal physical movement, minimal abrasion, minimal dulling of edges.