Sit next to someone working a shared inbox for an hour and the pattern is obvious. Opening and reading each message takes seconds. Everything after that is the real bill.
Each email forces a chain of small decisions. Who owns this. Is it urgent. Did someone already reply. What do we even say back. None of those decisions is hard on its own. All of them interrupt the work the person was actually hired to do. Run your own math: messages per day, times a few minutes of deciding and forwarding each, times whatever you pay the person doing it. It adds up faster than anyone wants to admit.