Knives are normally used to cut softer objects. So the edge of the knifes blade is harder than the substance it cuts through.
A blades geometry is more complex, but let's simplify it to the blade getting thinner towards the edge, ending at some thickness with a roughly round end.
So a hard substance pushes away a softer substance. I can see that on the first use, some irregularities break away, but it should become stable soon.
If the hard substance, like steel, has a crystalline structure of small crystallites, some of them may break away - but that is not fundamentally different.
But if a knife becomes dull on repeated use, the hard surface must be eroded somehow. Is that caused by grains of hard substances that are present as contamination in the soft substance? Or does a soft substance gliding over a considerably harder substance some other side effect other that gliding?
Why do most or all knifes become dull from repeated use?