[deepnotderp]

More rotors aren’t more efficient: the more rotor area you have, the more efficient.

Whether you achieve that with 4 small rotors or 1 big rotor is immaterial.

The reason eVTOLs use many small rotors is because an eVTOLs only operate in vertical mode for a short period of takeoff and landing. They aim to be normal fixed wing aircraft for most of the flight!

A helicopter like big rotor would be inefficient- adding weight and drag during forward flight. Helicopters on the other hand, spend the entire flight in vertical mode, for which a big rotor is most efficient.

this stems from newtons second law, but is neglecting second order effects like interference