These subreddits are big enough that their userbases are converging toward a statistically-normal sampling of the world population. To the degree to which the world contains prejudiced people, so will large subreddits. To the degree to which the world contains groups that venerate prejudice, prejudiced comments on large subreddits will be upvoted.
The big difference between a large subreddit, and the world-at-large, is that the majority cannot hang the implicit threat of mob violence over a disliked minority. In the world-at-large, disliked minorities usually meet in secret because they fear what would happen if they were outspoken in the presence of the majority. On a subreddit, there is no such fear to throttle behavior. As long as that fact remains true, large open online communities will always be outspoken-minority cesspools.