Right, but neither "milk" or water" is the plural of "drop."
I don't see how that really makes much of a difference, besides confusing the issue.
Or rather, it's the fact that
hair (substance) and hair (individual element) look and sound the same that confuses the issue.
Let's suppose that we spoke a dialect where the individual elements of hair were always called
strands and never of hairs[/i]. In that case hair would be exactly analogous to milk.
drop : drops : milk :: strand : strands : hair
You can see in this situation what I meant by "Hair is singular for the same reason that milk is."
I don't think that changing the word for the analogy to drop from strand to hair changes the fact that milk and hair, when used as substance nouns are both singular for the same reason. Because they're both substance nouns.