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.