Here's why:
* Indefinite pronouns refer to unspecified people or things. Examples include: someone, anyone, nobody, everything, etc. They don't have a specific identity.
* "Man" is a noun, specifically a masculine noun referring to a male human. It has a specific identity (a male human).
However, in some cases, "man" can be used in a generic sense to refer to humanity as a whole, regardless of gender. This usage can be considered a singular pronoun representing all people. This usage, however, is often criticized for its lack of inclusivity.
It's important to be mindful of the potentially problematic nature of using "man" in this generic way, as it can be seen as excluding women and other genders.