To some extent, it's probably a bit historic - back in the day, you would almost certainly not connect an SWA to the motor terminal box due to vibration issues - the motor would be connected by flexible conduit. As flexible conduit could not be relied upon to act as a CPC, it was a regulation requirement to offer a CPC and it became common practice to run a CPC on the outside of the conduit, which picked up the terminal box and the motor frame.
Over the years that's translated into one of those "Thou shallt……….." issues
