Technically they do. The US units are defined by the metric system, and they are full members of the Metre Convention since 1878.

You have 20 apples that you put into 5 boxes. “How many apples per box?” 20/5=4. Easy.

You have 20 apples that you put into half a box. “How many apples per box?” 20/0.5=40. You can see how 20 apples in half of a box implies 40 apples per full box.

You have 20 apples that you don’t put into anything. “How many apples per box?” 20/0=? What box? What are you talking about? How can you suggest that there are a number of apples per box when there’s no box to begin with? Lunatic!