Many good players intentionally wipe if a try is botched for one or multiple players because of logs or simply because they don't consider the resulting kill smooth enough, even if they could still clear it. That's a matter of choice, not difficulty.
Also, mistakes happen because humans are.. well.. human. Even if you know a fight really well, a momentary zone out is all it takes to fail a mechanic you would usually easily play correctly without anyone reminding you to and that, in turn, often snowballs into more mistakes or deaths because of how the fights are designed.
Many mechanics aren't particulary difficult. You have patterns to follow, fixed positions you get assigned etc., it's mostly just a matter of memorizing and the boss is the vocabulary test. There is little "winging" involved outside of first week when nobody knows what they're doing yet.