There is a good engineering reason not to. Small aircraft performance is very sensitive to weight, a typical small airplane will only have a non-fuel payload of around 1/4of the total weight. The added weight to make the aircraft also function as a car (removable / pivot-able wings, road worthy tires, bumpers, transmission, etc) will rapidly cut into that weight and reduce the overall efficiency.
At the same time modern consumers have become accustomed to very well optimized car designs, the added weight / drag of the folded wings etc will reduce the automotive performance to a point where it is not competitive. There are also significant differences in the optimization of automotive and aircraft piston engines.
Then there are operational issues: Aircraft require a thorough pre-flight before operation, so there is a significant delay at the airport, aircraft are almost never a time saver for trips of less than an hour driving. Most modestly priced (1M$) aircraft have limited weather capability and have nothing like the reliability of cars for transportation.
The real killer though is that (at least in the US), most small aircraft pilots will drive to their home airport, transfer their luggage to their plane, fly to the destination and rent a car. Rental cars are generally delivered right to your aircraft when you park for a small fee. In this way the airplane is optimized for flying and for a similar price as much better performance than the hybrids.
There may be a few special purpose cases were a flying car will work, but with present day technology they are very rare. This is supported by the existence of roadable airplanes since the 50's, but none have seen significant production. The "flying cars" that do not have wings are not practical - not that none of them have actually flown, the pictures are just CGI. The power to weigh requirements are not possible with today's technology.