They/them/their was the singular neutral until a bunch of old white guys got together in the 16th century (or 1600s?) and decided to change it to him/his/he, and so it has been since.
A friend of mine said, "but it's so alienating to women!" and I kinda laughed, but I've been reconsidering recently just how alienating it is to read "he" and "him" and "his" everywhere in academia. It's used because it's "less specific," by convention; to refer to a man can be to refer to an abstract human, whereas to refer to a woman is to refer specifically to a woman. Or so they claim.
But I've been reading things directed to "she" and "her" etc. And after the initial weirdness (and isn't it telling that it's weird) I started to like it. A man reading something to "she" would probably feel like it wasn't directed at him, or would feel like the usage was awkward, alienating. So why shouldn't a woman reading he/his? Convention? Women have been exposed to this for the past several hundred years, to the point that it's no longer strange (well, at first they were actually excluded from academics... how's that for alienation?)
That's like saying, "I'm used to being ignored. It doesn't hurt anymore." You don't noice the hurt because it runs so deep it's become a part of you.
An example of how deeply it pervades our abstract, academic thinking: In my philosophy class this weekend, we were talking about social justice. We got on the topic of race, and my teacher said how awesome it was that we have a black president, "Because now black parents don't have to tell their kids they have to become president. It will be a given, like it is for white kids."
Cough cough cough. White kids, in the abstract, meaning many he-s, sure. Little boys probably don't have to be told (although it probably doesn't hurt.)
I said, "White male kids."
And he countered with something about Hilary, which is valid, but... (insert argument about women gaining power on male terms.)
I'm not a kid anymore. This is an ingrained part of my personality, now, and I'm just realizing how deep it goes, and I don't like it.
I should have said, "My parents told me I could be president when I grew up. I stopped believing it about the same time I stopped believing I could be a princess."
(Which was shortly after I stopped believing I could be a tiger.)