> but the use value of your tenth car is much lower than having those same ten cars (and thus the same productivity) spread across ten people.

But the point is that any (however small) value for the person who earned it is justified, while whatever value it might have for other people is irrelevant, since it's not theirs.

In the same way you could argue that buying a car in the first world is immoral because buying it for someone else in the third world would provide them with higher value.

Earned wealth is not public wealth, unless we live in communist utopia where nobody earns or owns anything.