[rsynnott]

Most don't. I can't see how the claim (ebikes are more energy efficient than normal bikes) could possibly be true in any other circumstance, tho.

[type0]

Some electric skateboard do. I think what's stopping many to use it more generally is that you would need to disengage the system if you want to use your bike without the battery.

More complex braking system might introduce a lot of problems to a bike where that should be a simple thing, probably easier to have extra set of hydraulic brakes in case the battery is dead.