Why does a helicopter have a large spinning rotor above its cabin?

Answer: Its rotating blades push air downward to provide the lift that supports the aircraft

The roof is the strongest part, so spinning there reinforces the aircraft — Structural strength alone does not explain why a helicopter needs a spinning rotor above its cabin. Its mounting must be strong, but the rotor's job is to move air and provide lift. A strong roof without working rotor blades would not hold the aircraft up.

Its rotating blades push air downward to provide the lift that supports the aircraft ✓ — The rotor above the cabin acts as a set of moving wings, pushing air downward and producing upward lift on the helicopter. In a steady hover that lift balances the aircraft's weight. Its large swept area lets it move air without the whole aircraft having to fly forward.

Its spinning weight balances the cabin and prevents the aircraft from falling — The rotor's spinning weight does not stop the helicopter from falling. Spinning objects can resist changes in orientation, but that is not a sustained upward force. The rotor supports the cabin by interacting with air, not simply by balancing mass.