Bleach, Calgon, Dr Beckmann — all based on active oxygen. Active oxygen handles general dirt, limescale, and residue. But not the invisible biofilm barrier and pet hair buildup.
Detergent residue, moisture, hair, dander, and skin oils form a protein layer. Active oxygen simply slides over it.
That's why the smell always returns.
Furthermore, the biofilm produces antibodies in response to these external agents. Feeding itself even more in places you cannot reach manually, such as pipes and the back of the drum.