Why landing a Hurricane is not carrier qualification

If RAF Hurricanes landed on HMS Glorious, why could they not already operate as carrier fighters?

Because landing once is not sustained carrier operation; the aircraft still needed arresting gear and structural strengthening for repeated war-load launches and recoveries.

The Hurricanes recovered aboard HMS Glorious demonstrate that a land fighter can be put onto a flight deck, not that it is ready for routine carrier service. Operational qualification also requires repeated arrested landings, takeoffs, and handling at full fuel and weapons load. Naval conversion strengthens the landing gear and the surrounding fuselage and adds the necessary arresting equipment. A one-way emergency recovery does not prove a complete carrier capability.