There is nothing preventing cars with manual clutches from doing that. People going down hills with manual transmission also keep the car in gear (they teach yo

There is nothing preventing cars with manual clutches from doing that. People going down hills with manual transmission also keep the car in gear (they teach you that). And it doesn't matter as it is not much fuel compared to the fuel used when accelerating, where automatic transmissions are less efficient. If that is the only reason for automatic transmission to be more efficient, then i doubt it is.