Well, the idea of true gender neutrality in language is a fairly new one.
Many western languages have it worse off; French is very gender oriented, for example.
The thing is, when the languages were created, gender was a far more important matter to society than it is today. Such a gender-neutral pronoun will most likely not appear mainstream until the concept of gender becomes irrelevant. By that time, of course, humanity will have evolved into
Iain M. Banks 'The Culture' and we'll all be speaking Marain, not English.