It is because the -‘s suffix means two things, a contraption of is and the possessive form (the genitive case) of a noun. People tend to get into the habit of typing/writing ‘s whenever they think either of those, to the point of it being an unconscious action. As far as I know, there is no other noun or pronoun that does not end with -‘s. It is just evil when the correct -s ending is so close to the intuitive -‘s.