Not always the case. The last time we cruised, I priced out various options....keeping the car the entire time and parking at the port garage, returning the car prior to cruising and rerenting upon departure.
That time, I found that it was only $24 more to keep the car for the entire time, including parking costs!!! It was very nice to get off the ship, cross the street, get into our car and drive away.
But....that will not always be the case. You need to do your homework and run various scenarios.