It promotes fun. Why does everything have to be hypercompetitive? The idea of a "culture of mediocrity" is a complete myth because "mediocre" is a relative term. By definition, most people are always going to be mediocre. Suppose we all magically woke up with the skills of world first raiders. That skill level would then become the new mediocrity. Chasing this goal of "promoting excellence" in a video game is a ridiculous strategy that ultimately results in a game that no one wants to even play anymore.
This is how it plays out: Given enough practice, enough people will master any given mechanic. Once enough players have mastered it to the point that it's second nature, those players who were now considered masters become mediocre. In the meantime, the new players who have yet to master said mechanic are now considered "bad." Now that most existing players are easily clearing the bar, the devs have to introduce an even higher bar for them. As new players come on board and see ever higher bars, they eventually decide that clearing those bars is simply not worth the effort, and they choose to spend their relaxation time engaging in an activity that's actually relaxing.
Congratulations! You've mastered this game. Find another to master, if you're bored. Please don't ruin this one for the rest of us. You might have better luck petitioning Blizzard's dev team with suggestions like these. They seem to be all in on this notion that if it's fun, enjoyable, and/or easy, they should complicate it for reasons. All in the spirit of competition for what was originally touted as a cooperative game.
