Because hearing doesn't work like vision (for me, anyway). Although I still think it would be interesting to show someone a picture of a large blue dot and then replace it with a picture of another, slightly different blue dot to determine how accurately they could tell the difference. I read once where there are 32 different shades of color in cigar tobacco wrappers. The ones who can determine them all are the cigar makers themselves and few others. Experience would seem to reign supreme. Subtlety does not mean non-existent.

Cable differences are subtle and my ears need time to get used to their sonic signature. Even so, I didn't score perfectly but I would have done less well with rapid switching. Unfortunately, the rapid switching so common in the area of testing audio components (not that I can find much evidence of testing!) will usually give the null result, thereby adding fuel to the objectivists fire that "it all sounds the same". Too bad. The question of whether cables can sound different has been answered. The two remaining questions are "why do they sound different" and "are the differences noticeable enough to matter to the listener".