[Christ]

Because it requires a complex braking system that converts braking friction to electrical energy, storing it in capacitor or battery banks for future use, thus including an electric motor drive which can properly utilize the electrical energy.

Braking could, however, be used to charge the accessory battery, especially on non-computer controlled systems. Enough people use the brakes enough times that charging the battery would be a menial task, I would assume. It would safe efficiency of the engine, as well, since it wouldn't be driving an alternator.