Brand arbitrage is a mainstay of many affiliate marketers. Given that one can transform success with it into (large piles of) generally spendable currency, most don't spend too much time optimizing for (small piles of) gift cards.
Great point. I mentioned in my post that "there isn’t a huge amount of upside" financially, but I definitely got personal value out of just doing the experiment. I'd suspect that most of the truly lucrative affiliate opportunities are extremely competitive.