Flying cars: why haven't they taken off yet?

In 1940, Henry Ford said: "Mark my words – a combination aeroplane and motor car is coming." With flying taxis apparently on the way, it looks like he was right, but what a wait. Eight decades years later, "dude, where's my flying car?" is shorthand for any stuff "they" promised us that we haven't got. We have always wanted to fly, so, as soon as cars came on to the scene, we wanted those to fly too. Early blueprints for the US interstate highway grid even had adjacent runways ready for flying cars.