Flying requires more energy. In a world where energy efficiency is only increasing in importance, that's a big problem. Also, if everyone is commuting to work

Flying requires more energy. In a world where energy efficiency is only increasing in importance, that's a big problem. Also, if everyone is commuting to work by landing on rooftops, then streets will no longer be dangerous: rooftops will be because they will be crowded with drones dropping off thousands of people.