Why don’t they have regen braking?

Regen braking is a great way to recapture electricity; however not with a vehicle as lightweight as a bicycle. In an electric car, there is often more than a ton of mass to continue momentum and create the resistance required to recapture electricity - even then only approximately 5% is gained. With the low level of energy recaptured and the often uncomfortable level of resistance created to rewind the motor (almost like squeezing the rear brakes as soon as the assistance stops) - regen braking in electric bikes unfortunately simply isn’t worthwhile.