That may be true of the big subreddits. I moderate a small, niche one simply because I was visiting it every day for years and then they needed a mod. I saw it as a chance to improve an online space that I liked.
This is a common pattern with all organizations, but especially charities, non-profits, or volunteer roles. At first, the work is done by those who love that cause, and commonly some combination of
- the cause becomes prominent enough that there is influence or prestige associated with the role now, that it attracts power-seeking personalities
- the original founders are too burnt out to care, or clueless/trusting such that they get outmaneuvered by savvier entrants
Personally, I see it as the entropic drift of an organization away from an original cause or mission (order) towards a vehicle for the pure exercise power (chaos).