That's because he didn't have any large gauge stranded silver wire on hand ...
Warning: Spoiler! (Click to show)
In the interest of science, I tried the BFS "Big Box" 6 AWG copper cables for myself and found them to be surprisingly good, especially since the price was only 30¢ per foot (now that's a bargain!). While 6 gauge silver wire should do copper one better, keep in mind that DeWulf and Lumley were referring to multi-stranded copper conductors that add up to 6 gauge. Solid silver in 6 gauge is about 3/16" (4mm) in diameter and is more like a rod than a wire, but conceptually, it occurred to me that one could try bundling smaller gauge silver wires to approximate 6 AWG. Curious to see if this idea had any merit, I used what was on hand and attempted to make a set of 8 gauge silver cables using multiples of both 13 and 16 AWG, but found that the mechanics were not as easy as they seemed. This configuration was frankly messy and difficult to manage. In the end, I concluded that it was best to stay with a single strand of wire, regardless of its gauge.
Besides, we're talking about flexibility and durability for headphone cables that are moved a lot. Speaker wires on the other hand are hardly moved.
This guy prefers solid core.