Also consider the workload. There is far more going on than just letting one mod working full time cover the entire board. Even the alert system and members flagging issues for attention would be hard to keep up. Said person would have a hard time actually enjoying the board. He/she would be too busy reading all those issue threads to get enough context.
Far better to distribute the load to various people and let them work in whatever time they can commit. The alert system allows mutiple people to get the notices, which means whomever is available first can tackle it. The inefficiency of multiple people getting notices is less an issue because if it’s taken care of, it doesn’t take long to check the threads and move on.
This allows the mods to participate in the board for fun, tackle any issues they encounter, plus deal with a specific set of issues if flagged by the system.