“The conclusion must be that when the language of generic “man” is used, the thoughts and images that are formed tend to be of men and not of women. Thus, when we use “generic” male language for humanity, women and women’s lives, accomplishments, experiences, values, perspectives, and power are made invisible, and, to a large degree, lost.”
— Engaging the Bible in a Gendered World from the chapter “Image and Imagination: Why Inclusive Language Matters.”
This is a pretty concise way of explaining why some might be bothered by the “man” being used to describe “all people.” Because we create mental images associated with words it doesn’t matter the intent of the language here so much as the fact that we know man means something very particular in our culture. In a culture that is dominated by masculinity (where much of our language is masculine) this allows us to believe that gender is hierarchal. That men are more important or more valuable than women.