The point is how widespread slavery was, how deeply engrained in the economy and how widespread the benefit was. To take the car analogy, everyone in the family derives the benefit of car ownership and has a stake in maintaining that ownership, even if the car is only registered to one member of the household.
I see that point, and it makes sense to a degree. However, the same points could be made to the situation after slavery when most of the slaves became sharecroppers. It would seem to me the “widespread benefit” was really only to the large landowners and their families who benefited from cheap labor before and after slavery.