Large-scale urban air transport would also have to be entirely autonomous. Drunk/distracted driving is already a major issue and cause of death in the United State. In three dimensions and when failure of control can result in collision with any arbitrary part of any structure, this is obviously a non-starter. So getting the FAA to sign off means not only producing a design that must be, elevator style, mechanically essentially incapable of failure, but also getting them to sign off on an autonomous low-altitude flight system for use in dense airspace.