They also manage the review process (screen papers, find reviewers, communicate with authors and reviewers), make sure that papers are accessible long-term (decades to centuries), typeset manuscripts, print paper issues, and several other tasks.
But yes, reviewers are not paid. A half-decent review can easily take 3-4 hours, and a typical paper gets 2-3 reviews, so something like 8-12 hours of PhD-level labor which are not currently compensated.
But yes, reviewers are not paid. A half-decent review can easily take 3-4 hours, and a typical paper gets 2-3 reviews, so something like 8-12 hours of PhD-level labor which are not currently compensated.