The reason to avoid running your own forum is because the cost of effort to interact on a bespoke forum is lower than it is on a dedicated one. Subreddits usually ban people from posting who have under a specific amount of points; this stopped the illiterate from posting for a very long time, up until the parent company started pushing the mobile app and punishing subreddits that do. If you compare the discussion quality on a subreddit that still enforces quality posters with, say, Apple's bespoke forum, you'll quickly see why the platform is useful.