The problem with an airplane/car hybrid (besides the dual licensing requirement) is that you can't just take off from wherever. At least in the U.S., you need to take off from an airport, file a flight plan, etc. Otherwise, the big men with the stars on their chest pockets will be wanting a word with you.
There's also the Moeller air car, but it only flies, it doesn't drive. And it runs into the same issues.
The devil may be in the details, but hell is in the infrastructure. When the car appeared, it made use of the existing horse carriage infrastructure (which caused some controversy, but they were freer times.) When the plane appeared, there was no flight infrastructure to impinge upon. But a flying car today must fit into both the existing car infrastructure and the existing flight infrastructure - which are well entrenched, heavily regulated and protected by numerous interests.