I suspect the OP is alluding to an earth cable by its self buried in a wall outside of the "safe Zones", and if that's OK or not.

He's possibly asking why an earthed conduit may be buried outside of the "safe Zone" (with twin and earth in it, i assume) but an earth by its self can't be buried outside of the safe zone similar to earthed metal conduit. 

My reply to this assumption on my part, would be, the earth cable (bonding cable perhaps?) buried in a wall outside of a safe zone, while electrically safe, is very poor practice and, in future, could more easily be cut by a wall chaser for example or damaged some other way and possibly be partially or completely disconnected in future works. Just because the original installer was too lazy to run it neatly and in the expected zones, where an electrician might expect cables to be.

I'd not be happy with an earth run outside of the expected zone of where one might expect to find cables. 

The metal conduit would be very difficult to cut without knowing you've cut it but a copper cable you can easily cut and never know it with a wall chaser. 

Just poor practice rather than against the regs I suspect. 

I suspect the OP is alluding to an earth cable by its self buried in a wall outside of the "safe Zones", and if that's OK or not.

He's possibly asking why an earthed conduit may be buried outside of the "safe Zone" (with twin and earth in it, i assume) but an earth by its self can't be buried outside of the safe zone similar to earthed metal conduit. 

My reply to this assumption on my part, would be, the earth cable (bonding cable perhaps?) buried in a wall outside of a safe zone, while electrically safe, is very poor practice and, in future, could more easily be cut by a wall chaser for example or damaged some other way and possibly be partially or completely disconnected in future works. Just because the original installer was too lazy to run it neatly and in the expected zones, where an electrician might expect cables to be.

I'd not be happy with an earth run outside of the expected zone of where one might expect to find cables. 

The metal conduit would be very difficult to cut without knowing you've cut it but a copper cable you can easily cut and never know it with a wall chaser. 

Just poor practice rather than against the regs I suspect. 

"Just because the original installer was too lazy to run it neatly and in the expected zones, where an electrician might expect cables to be."

This is not always possible.

As it happens, in the instance which brought this up, I was able to drag the new earth cable through the path taken by the original earth cable (which needed renewing), but it was by no means a given for this to be possible.
With a power cable, you can always get around  safe zone requirements by installing a socket to open up new allowed pathways. You don't have this option with an earth cable. Had it not been possible to reuse the same route, in this occasion the new earth would have required to be surface mounted which is going to be a hard sell to many home owners. Particularly when had it been a T&E multiple options for routing outside safe zones exist.

Which is why I believe standalone earth cables are an oversight in the regs.