I think, correct me if i am wrong, that cars are easier to make then other vehicles like planes and especially helicopters.
I say this because there are more factors you will have to tweak for flying vehicles.
Second of all, it is easier considering there are so many real life car models at this current time to base their models off of.
It is probably easier to get your inspiration for a car model than a flying vehicle like a plane or a helicopter.
I would like to see more flying vehicles myself as i am a huge fan of them (even though i cannot fly them properly myself haha).
The Heighcontrol mod is in a huge change currently which will hopefully make flying helicopters a bit more realistic/fun while as well giving modders an easy way to use heightcontrol for other things, like the submarine mod (which in my opinion surprised me enormously and was a lot of fun :D).