Because when an outlet is active and something is in it, the electric current is just sent to the very end of the charger which would be considered using the electricity and you’ll be charged for it.
Power is only consumed if the device is drawing power. Transformers which go between the wall and a device draw power and convert it from AC to DC. That process consumes power, even if the device the converted power is destined for isn’t using the power. That is why transformers get hot and why you should unplug them when not in use.
Devices which use straight AC power don’t consume power unless they are on.