I totally understand your frustration with this limitation, it bothers me as well.
The reason this hasn’t been added is, to be honest, architectural. Octavox was initially built as a diatonic pitch shifter, and MIDI support was added after the initial release of the plugin. Adding chromatic support would be quite difficult without overhauling most of the plugin. So, it’s unlikely this will be added to Octavox.
That said, we understand how/why this is such an oft-requested feature, and hope to address it in some way in the future.