Because your assumption that “‘man’ already refers to both male and female humans” is incorrect.
The primary definition of “man” is an adult male human. The fact that “man” or “mankind” has historically *also* been used to refer to both male and female humankind is representative of the lack of importance that male-led societies have traditionally placed on females and their role within those societies.
So as to not perpetuate that misogynistic use of the word, someone who cares about such things will use the term “man and woman” or “humanity” or “humankind” to be inclusive, rather than glossing over and excluding 50% of the species.
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