Same thing that’s clearly been heavily at the forefront of their developmental mindset since Warmind. They know they don’t have the volume of content or depth of replayability in content to ensure players will stick around long enough between expansion releases, so they’re using one of the simplest design tactics possible and just using heavy RNG to artificially bloat the time it takes for players to complete things. It’s not particularly rewarding or balanced and does annoy the hell out of a lot of players, but it’s successful enough at keeping people playing that they don’t see real pressure to significantly move away from it.