Regulations could be one reason. Perhaps you can for your car over bodies of water, but not over land. Since it is impractical, and even less efficient, to have

Regulations could be one reason. Perhaps you can for your car over bodies of water, but not over land. Since it is impractical, and even less efficient, to have three vehicles (car on each side + a plane) then a flying car is better. You may be right technically, but there are practical reasons why flying cars may not travel over land for a very long time. I suspect noise is one of those reasons, it's why planes are limited to certain speeds despite being able to travel faster at higher efficiencies. It would be a great disturbance to the residents living below.