I guess the human body is much more efficient than any battery/motor.
The issue is really that if you're using a motor to capture energy that comes from human leg-power, you are ADDING the energy-losses of the motor+battery to the energy losses that were already there. If you just don't use a motor, you don't lose any energy in the motor.
Apparently humans can convert food energy into motion energy at about 20% efficiency.
If you consider losses along the system of fuel => powerplant => electric wires => charger => battery => motor controller => motor, the over-all efficiency is much lower than 20%.
If you then consider the fossil fuel energy that goes in to creating the
average USA person's diet, the efficiency for the human is also far worse.