A motor when physically turned produces power in the same way as an alternator, so adding an alternater just adds %loss through heat and added weight. Also in a gas car, the engine always runs like a generator to turn the shaft in an alternater. EVs already recapture generated electricity through rolling and braking. But there is still %loss through heat, light, computing power, friction. So you drive up a mountain and then coast down, you won't regain all the power, but you do get some.

As time goes on, better wiring, battery and even an addition of a single cylinder engine can be used to create and store power for the batteries to drive the motor. Innovation takes time, but tomorrow will be better than today. Even in recycling of batteries :)