probably because it wouldn't be a secure storage.  p9p is hosted on linux or macos, and if the keys are stored in a file (or device) on the host os, then secstore would be only as secure as the host os (e.g. sudo privilege).

readnvram (src/libauthsrv/readnvram.c) expects to find the keys in locations that are specific to plan9 devices (e.g. '#r/nvram'). unattended startup of secstore assumes that there is physical access control to the machine and only the host-owner can access nvram.