In my opinion it's the art and marketing teams finding themselves into an unholy alliance where they decided that showing insanely flashy visuals sell well (they probably do, just watch people reactions at job trailers). Then when you actually get to deal with them ingame they become a a huge detriment visually.

They used to be more grounded in earlier expansions since at every new expansion it's like they're trying constantly to push the bar further and outdo themselves, but let's be honest, bosses were already gigantic discoballs no matter what due to spellcasters.