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.
There are 20 houses on my block. The families have one husband, his wife, and on average 3 children. On average one child is old enough to drive. The families have a car for the husband, one for the wife, one for the child who can drive, and some have a RV or truck. All total there are 75 vehicles.
Because of the laws here, every home belongs only to the husband and so does every vehicle.
Using your logic, only 20% of the local population has a home or a car. This is in fact true. It is also a fine example of How to lie with Statistics. This type article comes out all the time, clearly trying to say slavery was far less widespread than it was. If it makes you happy to ignore this info, fine.