Member Rara Avis
For the same reason we don't say mentra when we mean mantras.
man [Middle English, from Old English mann]
"Usage Note: Traditionally, many writers have used man and words derived from it to designate any or all of the human race regardless of sex. In fact, this is the oldest use of the word. In Old English the principal sense of man was “a human,” and the words wer and wyf (or wæpman and wifman) were used to refer to “a male human” and “a female human” respectively. But in Middle English man displaced wer as the term for “a male human,” while wyfman (which evolved into present-day woman) was retained for “a female human.” Despite this change, man continued to carry its original sense of “a human” as well, resulting in an asymmetrical arrangement that many criticize as sexist." (Emphasis added.)
The etymology of human, on the other hand, can be traced back to Old French humain and Latin humanus, which are both ultimately derived from the Latin root humus, a word literally translated as "earth." A human, therefore, was "of the earth," as opposed to the gods of the heavens. Interestingly, and perhaps ironically, the Latin humus is also the root for our English word, "humble."
Tracing the origins of words can be fun, but as in most things, we should avoid making hasty assumptions. The her in Hercules didn't make him a woman, the his in history has nothing to do with gender, and the man in mantra relates not to humanity, but rather to God, in its Hindu roots.