Even if air taxis can be made to fly safely, there's nowhere for them to take off and land. For safety they would need an open space free of trees, tall buildings, and overhead wires. How many of those spaces do we have in dense urban areas?

And they're generally not going to land on rooftops either. A roof has to be designed with a helipad from the start. It's tough to retrofit one later due to structural issues, and use of roof space for antennas and machinery.