Flying vehicles are expensive: to build, to operate, and to maintain. Probably close to an order of magnitude for each, though the last may be close to two magnitudes.
Cost per pound of cargo in an airborne vehicle is huge. Here's an example: the HondaJet runs around $5m. A comparable Honda Odyssy is $35k.
Flying is a significantly more energy-intensive operation than rolling along the surface. A cheap prop plane like a Cessna has an airspeed around 100mph, and gets 20mpg. The equivalent super-cheap subcompact doesn't have to worry about winds, and gets 40+ mpg. Turbofans (like the HondaJet) which you'd want on a "flying car" get 4x or worse gas mileage than a Cessna. VTOLs (whether helicopter or directed thrust) are getting 6x or worse gas mileage.
The vast majority of aircraft require HOURS of maintenance per hour of flight time. Even the small, simple stuff like Cessna are more than a 1:1 maintenance:use ratio. There's not really any way to avoid this, since flying is significantly more hard on the vehicle than driving. So, for each flight you take, you're going to have to pay several hundred dollars in maintenance fees.
Overall, even with some reasonable improvements and economies of scale, you're looking at a vehicle that costs 10x or more than an equivalent wheeled vehicle to start with, and has an annual operating cost around 100x.
Besides, we already have flying cars. They're called helicopters. Notice how the pricing on those has kept them from be adopted. There's no real way to make a flying car significantly cheaper. And you're still stuck with the 1-6 reasons above.
Asking where flying cars are is only slightly more inane than asking why we don't have personal teleporters, and about the same as asking why personal jetpacks aren't sitting in everyone's closet.
-Erik