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

This is why some razors have instructions to wash your face before using.

You can use an old toothbrush to clean the edges of the razor to give them more life.

And some people believe that wiping the blade backwards on a piece of firm cloth like denim hones the edge so that one blade can last several months.

I've tried that several times. In some cases the blade does in fact seem to last longer. In other cases, there doesn't seem to be any improvement at all.

https://www.youtube.com/watch?v=gjSkgz3-2Ig

Could a razor blade be made that never dulls with a diamond or some other hard material (tungsten carbide)?

In theory a diamond blade will never corrode and will always be resistant to fine stone grit, but diamond is brittle = edges can get chipped. In theory a stainless steel strip could be coated with plasma deposited diamond and then sharpened = costs ya extra... One company sells them, there may be more https://www.cadenceblades.com/materials-coatings/overview/di...

I learned on Joe Rogan that ionic engines have one atom sharp tip, honed constantly by the plasma flow.

One atom sharp razor would be awesome. You will notice the cut only by the sound of your detached finger hitting the bathroom floor.

material kind's property, of course

martensitic steels, is a super-hard alloy, honed through heat and tempering, used in razors, surgical tool, ball bearing, some disc brake

Hardened steel-carbide alloy, this's less brittle,

etc. https://www.wired.com/story/why-do-razor-blades-dull-so-quic...

Same reason any blade becomes dull with use: cutting things changes the shape of the cutting edge, and not for the better.

The edge is microscopically thin and our human hair is abrasive. All material eventually dulls.

What kind of asinine question is this?

Because of entropy?