This is an entirely new convention that is just getting started and isn't universally accepted.
The reason it is not sufficient to just state the subject pronoun is that pronouns are are not just about identifying which of two binary genders a person belongs to. Not everybody wants to be he/him/his or she/her/hers. Some might want to be (to quote some examples) ze/hir/hirs or ey/em/eirs. Unless or until everybody is familiar with all of the possible variations, you will want to be able to specify all three pronouns. (There are four in some cases, but three is usually enough).