Randomness wouldn't simply work in a game like this, if they added went away with scripted patterns they'd have to forgo skills that are able to kill you if you get hit, meaning it would make content pretty much trivial to any static group, else it would make absolutely frustrating, if some people can't deal with mechanics even if they know when they're coming, them being random would make the problem x100 worse and the game would be based around luck... unless they make the bosses way less powerful and not able to kill you if you do a mistake.

Intricate bosses and randomness can't go together, you'd end up with either very easy bosses or luck based frustratingly ones.