Because there isn't an easy way to implement features that we want. InteropUnlock apps (as of 1/18/14) only allow the rebuilt YouTube app to upload videos from your phone, allows SamWP8 Tools to change a few system settings, allows WP8 Webserver to read more of the Registry and File System (AFAIK, the rest of the device,) and WPH to change other settings.
Individual volume control is unknown now because we simply don't know what's in all of the unofficial API and a few similar issues. Sure InteropUnlock has been around for a few months (5?) and we simply don't know what all isn't available.
For custom ROMs, Secure Boot would essentially need to be disabled in the UEFI, BitLocker world be a second challenge.... No recovery key, no username and password.