If you actually examine the structures of a bootable CD or DVD, you will find that there is a boot environment which loads DOS or WinPE and mounts an appropriate driver for the optical device, and then restores the Ghost image from what is effectively a separate partition. Ghost files in themselves are not bootable.
Your solution in creating a bootable USB flash drive and then placing a Ghost image on it is the way that everyone else does it, as far as I am aware. Unlike CD or DVD, usb devices are read write and are treated differently by the operating system, hence the solution has to be different also.