[spollock]

Who says they have to use battery tech? On a human scale drone I'm sure they could find room for a gas tank.

[maaaats]

For a quadcopter you need very fine control of the rotors, making a combustion engine hard to use. So then you will need to do combustion -> dynamo -> electricity -> electric motor.

A helicopter is controlled in a completely different way.

[rebootthesystem]

Larger multicopters are free to use alternative control methods. The method used in toy multicopters is both inexpensive and cheap yet other options do exist.