Mods get a lot from moderation other than money, maybe money too in some cases. If it was truly without any returns, nobody would do it. They have disproportionate power to ban anyone immediately. They can strut around with their pinned posts, "warnings" to the communities, content deletion. They get to play the community lead despite having unknown credentials for it. Because of that, the quality of moderation is very low, unbalanced, and disproportionate. I think Reddit's moderation structure is cost efficient but suffers from quality and consistency.