As several answers have said: flying cars.
The reason is interesting because it isn’t a failure of technology.
- Flying is an inherently dangerous activity. Nobody survives a fall from 1,000 feet, much less 33,000 feet.
- A trivial fender-bender on the ground would be much more damaging if rotor blades were involved.
- If you fail to top up the oil or check the tires in a ground-car you might get stiff bill. In the air, you might die.
This is the reason that new drivers can pass their driving test in a week but pilots have to log hundreds of hours before they are allowed to fly solo. This is the reason that planes cost many thousands of pounds/dollars/whatever.
There have been flying cars in some form or other for almost a century but they have always been rare and expensive because their owners have always needed a pilot’s license. That means there has never been an economy of scale; they’re not mass-produced like cars. That’s not going to change until we get totally autonomous robot pilots that don’t need a human to monitor them and enough self-diagnostics that equipment failure almost never happens.
Here’s the autogyro. Invented 97 years ago. Short take-off and a landing requirement that approaches zero space. Cheap and inherently safe. But still needs a pilot’s license and enough space to swing a rotor.
If you are thinking, “you won’t get me up in that thing,” that’s why flying cars are not around.