Keep in mind that ze/zy/zo/zum (and the similar shit the real SJWs do) would be the different cases of the pronoun. English pronouns usually have a long sound for the subject case and a short sound for the object case, so they want their neopronouns to match that. Unfortunately they usually only list 3 cases, despite the fact that English pronouns have 5. The assumption seems to be that they all conjugate like "she". As for what's wrong with just ze, there's a SJW answer and a shitlord answer. The SJW answer is that having multiple sets of neopronouns lets people better express their true identities better. The shitlord answer is that all neopronouns are unnecessary and that if you must use something gender neutral, you can just use singular they.