Etymologically, in Old English, man used to mean person, and there were explicit terms for a (male) man and a woman - wer and wif. Wer fell out of use[0], and the generic "man" started referring both to all people, and to men (males) in particular.
This conflation found in early modern English uses of "man" started being seen as problematic in the early 20th century, and style guides started preferring other words to refer to people in general, thus relegating "man" to mean only male person somewhat artificially.
[0] the only remaining usage is in werewolf - though few would assume a werewolf has to be male.
[1] note that I am using "male person" only to make it clearer when I'm using the generic or specific meaning of "man". I'm not trying to conflate sex with gender or any other kind of subtle transphobia.
This conflation found in early modern English uses of "man" started being seen as problematic in the early 20th century, and style guides started preferring other words to refer to people in general, thus relegating "man" to mean only male person somewhat artificially.
[0] the only remaining usage is in werewolf - though few would assume a werewolf has to be male.
[1] note that I am using "male person" only to make it clearer when I'm using the generic or specific meaning of "man". I'm not trying to conflate sex with gender or any other kind of subtle transphobia.