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@coolandnormal nah, you totally could
Basically it's efficiency of manufacture
Ebikes don't have regenerative braking, because the motor is separated from the wheels by a freewheel system. The freewheel is usually in the rear wheel hub, because this allows them to use a standard bicycle hub. This means the motor cannot be driven as a generator. This also means they don't have to bother with designing the motor to handle being used this way
So to charge the bike you would need a separate generator system that attaches to the bike, like an indoor trainer
So they're not built to do that, because they use standard parts

Apparently such generators do exist, although I don't know if your ebike can support charging from a couple of hundred watts