Corrosion coupled with abrasion. If people washed their faces well in advance, they would eliminate dust in the facial grease. This dust is only partly hard particles - silicon dioxide from rock and other hard stuff. Skin and soft dust is less abrasive. This abrasive part is harder than steel and scratch by scratch wears away the edge. This is well documented. The other wear factor is corrosion - hardened steel corrodes and adges get smaller. Stainless steel does not corrode, but is softer and abrasive wear dominates. Eventually the feel of a blunting bladed prompts a change, they grab/dig/cut into the face