Coverage shape
A turnaround is a fixed sequence of tasks against a fixed block of time once an aircraft is on stand. Miss the window and the delay is immediate and visible, not absorbed quietly the way a quiet control room can absorb a late relief.
Qualifications
An airside driving permit, a dangerous-goods certificate, a de-icing qualification, a marshalling sign-off. Each has its own validity period, and several require a periodic practical reassessment rather than a one-time exam.
Statutory rest
Ground staff answer to general working-time law rather than a flight-crew-specific fatigue rule. It is still a duty-length and rest constraint an engine would have to enforce, whatever instrument states it for a given country.
Demand pattern
Demand tracks the flight schedule, not the clock. A bank of departures at the top of the hour needs far more ramp staff than that hour's average would suggest.