Human effort just isn't worth very much. The strongest humans on earth can manage about 400W for an hour. Even very small ebike motors are usually capable of 500w continuous/forever, so long as the motor does not spend too much time stalled or at very low speeds.
For a normal human, a "legal" 250w motor is easily doubling or tripling their normal power output, so hauling around an extra 20kg on top of the existing ~100kg body + bike is not a big deal.