The CPC (earth) is not a live conductor, so it does not need insulating in the way live and neutral do, as the earth sleeving is an insulator. It needs protecting from its surroundings, not protecting someone from the cable.

Do you need to sheath an earth wire?

The earth or "ground" wire is normally bare. The wires are then jointly covered by the outer sheath. ... When installed, bare earth wires should be covered with an "earth sleeve" which can be bought in all sizes at an electrical store or most diy stores.

Why do you have to sleeve earth?

What you call sleeving is, in fact, insulation. Since the circuit-protective conductor is not a live conductor, it doesn't need to be insulated - it only needs to be protected from the environment (which it is, by the cable sheath).