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)"
Best Answer
You pose what I take to be two questions:
(1) Why is 'head hair' two words instead of one (especially given other words like bedroom)?
We all know what a car radio, a toaster oven, a graveyard shift and a spring chicken are; I don't think we'd benefit from making them a single word, even if other languages might do so -- indeed French and German have 'autoradio' instead.
'Word count' is really about language-specific word derivation practices, practices that might be constrained by grammar or just by custom.
The more interesting question you pose is
(2) Why is there no separate lexeme for head hair?
Well, as others mentioned, there are rare words like 'chevelure' and 'coif' that may fit the bill; on the other hand, they might be better viewed as foreign words. In any case even without them I think we need to remember that vocabulary does not develop merely as a result of 'need': there is a lot of randomness in language (one might draw an analogy to 'genetic drift' in the theory of evolution, which results in random elimination of some genes in a population, merely as a result of chance (http://evolution.berkeley.edu/evosite/evo101/IIIDGeneticdrift.shtml)).
I personally think that English has a lot of vocabulary that is not 'needed' by any objective, non-sentimental criteria, since other languages seem to make do with paraphrase in the same situation (and likewise for other languages). Vocabulary seems to develop by random acts of creativity that are not especially 'useful' (slang is a perfect example, which lives and dies on sociological grounds, rather than on 'being unable to express oneself otherwise', although I accept the distinction between the two is not quite that clear-cut).
PS The word for 'gloves' in German is 'Handschuhe', or 'hand shoes'. No separate lexeme.
In brief, I think it is mostly because of chance that there is no separate lexeme for head hair.
Good evidence against this view would be evidence that, for example, discussion of head hair was taboo among prior English speakers (for example, because it invited the wrath of God. Many religions do still have hair taboos). A similar phenomenon is believed to have occurred for 'bear', which in many languages is derived from a circumlocution. In Croatian the word for 'bear' is literally 'honey-eater.' Even English's 'bear' is derived from 'brown.' The reasons are thought to have to do with warding off bad luck by avoiding a direct, separate name. See http://www.cloudline.org/LinguisticArchaeology.html.
But I know of no evidence for such a theory as regards head hair, nor even for the simpler theory that English speakers thought of head hair in a different way (a culture-determines-vocabulary type argument).