There are loads of things you guys have missed out like flight computers, navigation computers, external control surfaces, pressurisation, retractable wheels etc etc, none of these are needed in a car. Not only this but most systems on an aircraft are repeated 2 to 3 times in case one of the systems malfunctions, this is called redundancy.
I think the technology needed in order for an aircraft to fly is slightly higher than that required to stay on the road, the hardware and associated development costs for this would be far more expensive thanthat for a car.