Why flying drones has 4 rotors but not helicopters?

4-rotor copter is simpler and better stability. I suppose real helicopters don't because to save parking space??

Basically:

• Big blades is more efficient than multiple smaller ones.
• Electric motor can change speed fast. But combustion engines cannot.
• Single combustion engine is more practical and efficient than 4 smaller combustion engines.
• Real helicopters cannot use eletric for motor, because they are not efficient enough yet. Battery too heavy, and don't last long.