A white sneaker looks its best for a short window and then declines on several fronts at the same time. Each part of the shoe is failing in a different way, which is why one cleaning method never fixes all of it.
The sole yellows through oxidation
Rubber compounds react with oxygen and ultraviolet light over time. The reaction changes the material itself, producing the amber tint familiar on any older pair.
Because this is a change in the rubber rather than a deposit on top of it, scrubbing has no effect. The discoloured layer runs through the material.
Storage matters more than cleaning here. Sunlight and warmth accelerate the reaction, so a pair kept in a dark, cool place yellows measurably more slowly.
Leather and canvas soil differently
Smooth leather holds dirt on the surface and in the creases that form as the shoe flexes. Most of it lifts with a damp cloth and a mild cleaner.
Canvas absorbs. Dirt and dissolved substances move into the fibre, and once they are inside, surface cleaning removes only the part sitting on top.
This is why a canvas sneaker looks progressively grey while a leather one looks clean with dirty creases, and why the same product works poorly on both.
Optical brighteners fade
Much of what makes new white fabric look brilliantly white is a finish that absorbs ultraviolet light and re-emits it as visible blue, cancelling the natural yellow of the fibre.
These brighteners wash out and break down with exposure. As they go, the fabric returns to its underlying colour, which is warmer and duller than it looked in the shop.
The shoe has not become dirty at that point. It has lost an optical effect that was applied at the factory and was never permanent.
Washing machines cause structural damage
A machine cycle removes soil effectively but soaks the adhesives and foams that were never designed to be saturated and then tumbled.
Glue lines soften, midsole foam compresses unevenly, and the shoe often dries into a slightly different shape than it started with.
Water also carries dissolved dirt deeper into canvas and into the foam, which is why a machine-washed sneaker can look worse a week later than it did straight out of the drum.
Why the trend survives the maintenance
The appeal of a white sneaker is that it reads as deliberate and unworn, and that impression depends entirely on its condition.
Because the look degrades, the category has a naturally short replacement cycle, which is commercially useful and helps explain how persistently it is promoted.
Buyers who want the look without the cycle generally end up choosing smooth leather over canvas and accepting a yellowing sole as a property of rubber rather than a fault.