The short answer is the modern word woman has a separate etymological root than the word man. And the word man has had linguistic changes recently that mask those origins. Woman as well actually.
Man originally meant human really, and woman used to be Wifman (somewhat literally female human). Wife is a holdover from this prior to Middle english usage of the term.
Old english used wer and wif as man/woman respectively, and as you can guess Werewolf derives from the old ussage of man.
In short, its complicated and woman isn't a derivative of man. Its much more interesting than that. You can't look at the modern spelling of words to derive much on how we got to this point. You'll make a ton of categorical errors that way, especially in English given man used to just be the human race. Our modern take of man anywhere as needing to be replaced with woman depending on gender is slightly amusing once you know the etymology of the term in that regard.
Ditto "man" and "he" being used when the sex of the person in question isn't known, or as a poetic stand-in for "human"—the usual complaint about that is that it implies maleness is "normal" at the expense of women but it could just as easily be seen as erasing what's distinctive about maleness, at its expense and in favor of women, who get their own dedicated words that aren't multi-purposed and also used to refer to men in some cases. Just a matter of perspective who's harmed by it (if you take anyone as being harmed).
