in wall wiring , at least some of it that ive seen has not only the jacket around the individual wires as in all cables, but a second outer jacket which encases both the wires, this would add more protection, but also add more thickness.

when you lay the tile ( i dunno if youve ever done it before ) theres not alot of room under the tiles for something like a wire, you could run it through the grout line but that would most likely get ruined if you ever pulled on the cable or something.