The vehicles were meant to be secured.
In a classic love scene, Jack and Rose sneak into one of the Titanic’s passenger cars and have their way with one other, judging by the amount of steam in the car. But how were the two going to get one of the cars open?
In reality, there were cars on board the ship, but they were all locked. They didn’t want some random folks to get into their cars and contaminate them, for some reason. Unless car hijacking was one of Jack’s specialties, they’d have to get a room, as the saying goes.