The problem is this.
In order for the terrain to look "right" for the Aircraft, it needs to be dotted with things like tree's, bushes, hills, etc. All of which are hard to make look "right" for ground vehicles.
Getting this right for both Aircraft and Vehicles is hard, period.
If you think you can do it better, well go grab the terrain editor and have at it!
But I have to warn you, building a terrain is a long, hard, job. With virtually no rewards, and every one who flys or drives on it will find something wrong with it.