The mass of a bicycle and rider is relatively low, so there just isn't much energy to recover. It's not worth the more expensive motor and controller required to do it.
All ebikes on the market today already have the hardware for regen. You simply need to reflash the motor controller firmware to one which shifts the phase of the current by 180 degrees in the motor windings, and suddenly you're regen-ing. It can be as simple as a simple minus sign in the firmware source code, if only you had access to it!.
Some ebikes have firmware mods to do it - others not so much.
The main added difficulty is you can't recharge batteries too fast - and typical braking would produce too much power, and damage the batteries. If you didn't want to destroy your batteries, your regen would have to be pretty tame - more of a "gentle resistance" than "braking".
The mass of a bicycle and rider is relatively low, so there just isn't much energy to recover. It's not worth the more expensive motor and controller required to do it.
https://www.pandaebikes.com/regen-braking-ebikes-worth/