People with careers, families, friends and hobbies are mostly not going to spend their limited free time being a digital janitor for an anonymous online community.
People sitting alone in their apartment with nowhere to go and nothing to do and no one to spend time with, however, might find that being a Reddit moderator gives them a hobby, a sense of purpose, and feelings of power, importance or significance that they otherwise never get in real life.
Someone should make a social media site with inverted dynamics- users who only spend a few minutes per day on the site and post once every few weeks should be treated as the influential power users, while the people lurking and scrolling for 10 hours per day are deprioritized.