That's because hybrids aren't designed to do so. The battery is small in terms of both energy and power. Sometimes, if the car is initially pointed the right way, you could complete a very short downhill trip at low speeds without the engine starting. But hybrids are designed to run the engine often. The batteries are sized to capture approximately the kinetic energy of the moving vehicle when stopping, and discharge the same energy when starting to move again, and that's it. It's a great system, they all get 45+ MPG.