The motion of the car isn’t a byproduct, it’s the main thing you’re using energy to produce, and harvesting that same energy means you need to spend more energy than you’re getting back otherwise the car stops. That’s how regenerative braking works.

You could harvest waste heat for energy generation, but it’s hard and there isn’t really enough of it to bother with in an EV. They do dump heat gathered from the drive unit into the A/C system to save energy on heating the cabin though.