Harvesting energy from the front wheels of Formula 1 cars?

What is the biggest practical problem preventing harvesting braking energy from the front wheels in Formula 1?

One problem is that this would require drive shafts for the front wheels. This would all add to the weight of the chassis and would require space inside the monocoque. The rules also regulate the construction and as of now the aim is to keep the cars as simple as possible. Four wheel drive is forbidden.