Not a native speaker, but my assumption is that we are trying to go easy on “guys”, “man” and “he”. It is not hard to say “they”, “person”, “people”.

Also, I cannot really explain really well, but it seems like there is a big difference saying “he” about just mythical person, and referring to a real person (whose gender you don’t know).

Yep you’re 100% spot on. “He” works with hypotheticals (even if that’s a bit outdated) but not when talking about a specific person.

> It is not hard to say “they”, “person”, “people”.

Not hard, but incorrect.