Physics and practical concerns are way, way different. You want to go as fast as possible down the descents in a bike race. You don't want to lose any kinetic energy and fall behind your opponents, so the only time you'd be using it is when you actually want to slow down. In a car, you might be braking/slowing down going downhill anyway, so that energy is better captured than used that moment.

There's also the matter of mass: lot more momentum/energy to be gained from a 1500kg car versus a 70kg bike + rider. That said, less energy needed for the motor so don't know how the math works out there.

Edit: all of this is moot anyway because of the point zettabomb made as well.