Why doesn't English have a separate word for "head hair"? (head hair vs. body hair)

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).

Though it is of French origin, chevelure is a head of hair

Etymology: French, from Old French chevelëure, from Late Latin capillatura, from Latin capillatus having long hair, from capillus hair Date: 15th century a head of hair

If you refer to someone's "coif", (short for "coiffure") it refers to the hair on a person's head - usually styled nicely, as in "Her friends all envied her new coif", but it is most definitely a reference to head hair.

From MWO - the origin of COIFFURE is French, from coiffer - to cover with a coif, arrange (hair), from coife, from Old French

First Known Use: circa 1631