At first read you say why?
Why can I stuff 24" of pipe with wires and not derate?
I believe the answer lies in coppers ability to readily conduct heat.
Say you have a 24" nipple between a panel and a contactor enclosure, you will have conductors hanging out of both sides of the nipple acting like heat sinks limiting how hot the conductors in the nipple will get.
That's my story and I am sticking to it.
After all there has to be logic behind this exception.