Because they're both live
Just because you see multiple wires doesn't mean they are carrying different things. They are frequently paralleled for more ampacity.
It's perfectly common in HVDC lines to use earth as the return path. For instance, the Pacific Intertie does just that, with absolutely massive grounding electrodes at each end.
The grounding system at Celilo consists of 1,067 cast iron anodes buried in a two-foot (60 cm) trench of petroleum coke, which behaves as an electrode, arranged in a ring of 2.0 miles (3,255 m) circumference at Rice Flats (near Rice, Oregon).
The Sylmar grounding system is a line of 24 silicon-iron alloy electrodes submerged in the Pacific Ocean at Will Rogers State Beach suspended in concrete enclosures about 2 to 3 feet (0.5 to 1 m) above the ocean floor.
In that case, the multiple wires on the pole are all live, typically configured as redundant paths so the route is not entirely downed if one wire takes a hit.
The Konti-Skan HVDC line you are talking about does exactly the same thing, although it varies, using a number of methods. In some places it does carry the ground wire on the towers, but on top and insulated from the towers.
Before the implementation of Konti–Skan 2 both conductors were switched parallel, today one conductor is used for the high voltage pole of Konti–Skan 1, while the other is used by that of Konti–Skan 2.
The multiple conductors are either redundant or have different destinations.