Sure you can do that on a tiny place like NeoGAF, which still has a huge amount of moderators, open registration with a long waiting list, and a specific unifying topic--videogames.
But we're talking about tens of thousands of subreddits. Same with Twitter and FB. There's garbage posted on those, it's almost impossible to enforce, so they let the users self moderate, which is what I do with Reddit. I just don't visit those subreddits filled with vile commentary.
Comparing NeoGAF to Reddit doesn't make any sense.
Comparing NeoGAF to a specific subreddit, those. Each of those has their own moderation team.