Regenerative braking basically is doing that.

The probelm is the laws of thermodynamics. To successfully charge the battery using the kinetic energy of the car would make an EV doing that into a perpetual motion machine of the first kind. You’d be converting KE back to PE, to use to convert back to KE, etc ad infinitum.

In reality, you actually could “trickle charge” the battery using an alternator or something similar to draw a small amount of the energy, but due to the thermal efficiency of such a device not being 100%, as well as inefficiencies (energy losses) in the batteries, controller, motor, and wheel bearings, you end up losing energy compared to just using it to drive the car.

Say you feed 500W into this alternator, which is 99.9% efficient (that in itself would be amazing). Your controller is 99.9% efficient. Your battery’s charge acceptance is 99.9% efficient. The battery discharge is also 99.9% efficient. Of that 500W in, your battery is able to put 498W of it back into the drivetrain again — which would be remarkably efficient — but there’s an obvious problem…you’ve lost 2W to inefficiency. You could’ve just not drawn that 500W, and had 500W going through the drivetrain to start with.

Taking the car out of the picture, it’s trying to charge a battery by having it drive a motor to drive a generator to charge the battery powering the system. The end result is less energy coming back i to the battery than if you’d just used the energy in the battery without running it through anything.

Even more simply it’s like trying to charge a battery with itself.

Regenerative braking works to convert the KE to PE at a time when you’d be wasting that KE anyway — brakes work by converting KE to heat, which is lost forever. Compared to that, all the inefficiency of the system in trying to recover some of that KE is still a massive improvement.