Because at the end of the day, it doesn't matter if they make any difference or not. The fact that there are enough people that believe that they are effective at changing sound and having these debates will sell their products alone. And some of these people will even allow them to get away with their 10,000% margins. Hah.
I've had a few cables through my system and have decent stuff (nordost and audio quest). I like them because they're nicely made pieces, not because they make any bit of difference to the sound of my system. I don't have any problem with people liking cables for any reason, personally. I just think people should accept that they're probably experiencing a placebo and enjoy them for what they are - a designer, premium luxury item.
I sat in a cable demo at one of the big audio shows. It was laughable how many people followed along and just agreed with everything the sales guy said. I offered a bet of the $10,000 watch I was wearing if the sales guy could pick out his cables from the opposing brand 7 times out of 10 or more in front of the crowd. Of course, that offer was turned down. lol