Aircraft require significantly more space between them in order to operate safely so I doubt the efficiency is there even taking into account stacking in an ext

Aircraft require significantly more space between them in order to operate safely so I doubt the efficiency is there even taking into account stacking in an extra dimension. They're also pretty weather dependent in comparison to other options so capacity can vary a lot. If we actually cared about efficiency in urban transport we wouldn't have personal vehicles beyond bikes and small electric vehicles. We'd rather rely on public infrastructure like subways, trams, trains and buses.