Adding to the other factors already pointed out, some electrical motors (such as permanent magnet synchronous motors) actually consume energy when they are spinning but not powered.
Motors also have a specific range in which they operate efficiently in. Dual motor EVs employ different motors in the front and rear to take advantage of this and allow the car to operate efficiently at slow and fast speeds.
An ELI5 electrical motor and a generator are essentially the same thing, just depends on if you’re giving electrical power to in and extracting kinetic power (rotation) out, or if you’re giving kinetic power in and extracting electricity out.
Simplifying for ELI5, even if the motor was not connected to anything, the pure act of spinning the motor (such as when the wheels of the EV are rotating at high way speeds) will cause rotational resistance in the motor that will require more energy to overcome. This energy is converted into heat inside the windings of motor, which is dissipated and lost.
For a ELI16, the magnetic flux of the motor as its moving causes induced electrical current in the motor windings which, unless you’re playing with superconducting exotic materials, will converted to heat due to electrical resistance.
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