IIRC "ghost car" (especially under this name) got somehow granted SW-patent in US for some racing game (can't recall which one, but way before LFS existed), and the patent owner actually tried to enforce it few times over other games. I don't know if Scawen was aware of this, or he didn't add ghost car to LFS just because of lack of time/will, but it may be also he didn't want to risk any legal action.
(just googled.. it's Atari and "Hard Drivin'" arcade videogame, patents were filled in 1993)