1. Randomness has been disliked in the past.
Historically, fights featuring a high degree of randomness have been disliked by the playerbase. Prince Malchezzar and Faction Champions are the prime examples here. Both of them were fights which led to a high degree of grumbling.
I don't really see the point in adding more randomness, when the evidence shows that randomness is unpopular.
2. Randomness encourages "fishing".
Often, a fight with random elements contains one set of elements which is significantly easier or significantly harder than the others. This encourages guilds to reset the encounter until the "easy" combination shows up, or wipe it early if a hard combination appears.
My guild did this with Heroic Twin Valkyries back in Trial of the Grand Crusader. Twin Valks used one of four random abilities every minute or so. My guild used a very specific strategy that relied on rotating cooldowns to mitigate enough damage to survive. However, there was one combination of elements which that strategy could not handle because the cooldowns didn't line up correctly. For all other combinations, the strategy was easier to heal, but that one was a guaranteed wipe. So we fished. We just trusted in the odds, and took the wipe if it came up.
Honestly, though, it was kind of silly. We should have mastered a strategy that would allow us to always beat the fight, even if it took more time.
3. Execution is not trivial.
There is an unfortunate tendency in the geek subculture to regard thinking as hard, and execution as the easy part, the part to be left as a trivial exercise for the reader. I deeply disagree with this. Many times, execution is the hard part, and mastering the execution is the challenge.
It's like writing a novel. Sure, you can tell someone how to write a novel, come up with characters and a plot. But that's easy. The hard part is actually sitting down and writing out the novel, polishing page by page.
Mastering execution simply scratches a different itch for us. It's like learning to play a new song. Sure, one could say that it should be easy, because you've already got the music in front of you. But learning to play the song is still a challenge, still worth doing.
Joel Spolsky talks about trying to hire people who are smart and who get things done. In my view, smart people are a dime a dozen in our subculture. People who get things done, on the other hand, are exceedingly rare.
That's what raiding is aimed at. It's all about execution and getting things done. And it's still difficult, and still challenging. Just because you can't show off how smart you are, doesn't mean it is automatically an inferior activity.