It's a matter of the effort it takes to vandalize a community versus the effort to maintain it. Different communities have different thresholds for various things, of course. And, the more obscure a community, the less frequently it is vandalized, and the simpler moderation can be.
Classic reddit worked when it was small. Then subreddits, mostly to categorize, were added when it got larger. Then dedicated subreddit moderators were eventually needed and communities grew around subreddits. And then rules about invading subreddits were needed, and so on.