Regenerative braking on bikes is a bit harder than it is on cars - you can’t easily combine regenerative braking with a freewheel - if the rider stops pedaling that‘s not a sufficiently good signal to start regenerative braking. With cars, the signal is there - the driver stops depressing the accelerator.
Additionally, with middle motors (which most ebikes today have), you‘d need to have the chain run continuously to transfer the power from the wheel back to the motor, and you likely could not do that with a derailleur shifting setup.
That said, I‘ve seen (but not tested) one model that had regenerative braking, so that does (or at least used to) exist.
I can see the middle motor problem (though the company here claims to have solved it by using a front motor instead) - but there is a signal available to the bike; the brakes themselves.
I'm not sure that cars even begin regenerative braking until you actually hit the brakes as opposed to just letting off the accelerator.
The electric cars I have tried all started regenerative braking when you let go of the accelerator. As an additional problem for bikes: they are rear-wheel drive and the primary braking power comes from the front wheel for normal bikes (load distribution shift to the front when braking). Cargo bikes and tandems are a bit of an exception here. I’m not saying it’s impossible - as I wrote above I know it exists. It’s just a lot of effort for comparatively little gain, and so it’s pretty rare.
I think that's the main thing - the amount of energy lost by bringing a bike to a stop is nothing compared to that of bringing a car to a stop from 60 MPH.
And they're probably better off just adding a generator in-line with the pedals (in fact, I'm surprised these things have "direct drive" at all and aren't just a generator on the pedals with a wire to a motor on the front/rear hub).
Additionally, with middle motors (which most ebikes today have), you‘d need to have the chain run continuously to transfer the power from the wheel back to the motor, and you likely could not do that with a derailleur shifting setup.
That said, I‘ve seen (but not tested) one model that had regenerative braking, so that does (or at least used to) exist.