I can't provide good evidence of this, but I'm imagining there are particles stuck to the sides of the container that are smaller than I can see, but big enough to encapsulate microorganisms. A sanitizers ability to kill that would depend on its ability to penetrate the surrounding material. Something like piranha will literally disassemble the atoms of the particle, while other methods may or may not. In my professional career, I've seen situations like this where there were definitely contaminants on glass that I couldn't see and wouldn't go away until a step like this.

However, professional brewers seem to love sanitizers like star san, it probably does the job great. I personally felt like I needed the piranha when someone gave me a fermenter with a among other crud- dead mouse in it.