The answer can be "Because it doesn't!" or "It wasn't needed!" in short but there might be a historical or linguistic explanation behind this. (Of course, every language might be lacking a word that another language has and you can give the meaning in a context.)
This question came up mainly because there are several languages which has separate words for head hair and body hair. For example, head hair is capelli in Italian, saç in Turkish, cheveu in French and kopfhaar in German. But in English, hair is like a hypernym for head hair and body hair.
When we check the etymology of hair, the origin is related to the German word haar. And interestingly, German language came up with kopfhaar which literally means headhair.
Old English hær "hair, a hair," from Proto-Germanic *khæran (cognates: Old Saxon, Old Norse, Old High German har, Old Frisian her, Dutch and German haar "hair"), perhaps from PIE *ghers- "to stand out, to bristle, rise to a point" (cognates: Lithuanian serys "bristle;" see horror).
When we dive into biology, we can see that there are structural differences between body hair and head hair but they are composed of the same substance, keratin. Though, there is a distinction in terminology:
Androgenic hair, colloquially body hair, is the terminal hair that develops on the human body during and after puberty. It is differentiated from the head hair and less visible vellus hair, which are much finer and lighter in color.
And a twist ending: Wiktionary has a definition (and even an etymology) for "headhair":
From Middle English *heed-heer, from Old English hēafodhǣr (“hair of the head, headhair”), equivalent to head + hair. Cognate with Dutch hoofdhaar (“headhair”), German Haupthaar (“headhair”), Danish hovedhår (“headhair”), Swedish huvudhår (“headhair”).
I can see that "headhair" is used in a few sources but it is not common. Personally I haven't encountered this word before I did this research.
- How come both "hair" and "headhair" emerged (for the same meaning)?
- Why didn't "headhair" gain a common usage? What happened in the history?
- Does "headhair" have a current usage? (only in technical sources?)
Note: Just to emphasize, the question is not only about finding words, its about the history and origin of the words as well. Also, the question can be: "Why doesn't English have a common separate word for “head hair”? (head hair vs. body hair)"