1. Hard to fly
2. Have a problematic requirement for a long tailboom with a torque countering thrust at the end of it
3. Or counter-rotating rotors with complex drive requirements
4. Have rotors that are long and ungainly and need to be stowed
5. Need large amounts of power to generate all required lift
Making one into a car means solving all those problems, AND adding all the safety equipment etc that is required for a modern car, AND still having it light enough to get off the ground safely.
Fixed wing, Gyrocopter, or Paraglider based machines are a much easier task than a helicopter based flying car, as evidenced by there being actual existing modern examples of all three (Terrafugia, PAL-V, Maverick), and no existing examples of a helicopter based one.