The answer is multifaceted: 1) Because that's how people do it. Sometimes we all collectively settle on a standard that isn't the best, but that's just how it'

The answer is multifaceted: 1) Because that's how people do it. Sometimes we all collectively settle on a standard that isn't the best, but that's just how it's done. 2) It allows for more easily expanding to multiple pronouns, e.g. "he/they" or "she/they", which indicate that the person is ok with "she/her/hers" or "they/them/theirs". 3) It makes it easier to state neopronouns (e.g. xe), by standardizing the form -- he/him/his is the same format as sie/hir/hirs or xe/xem/xems. 4) (My opinion) It feels more natural to say "he him" than just "he". "What are your pronouns?" "He." I dunno, it's a short enough utterance that it could be easily to mishear?