Why dont they call them a neuter like any normal person would? That's what they are!
Because there are a helluva lot of people out there who are not neuters, but do not fall into either sex. And there are people out there who are perfectly fine as one sex and simply gendered differently. And there are people out there who are completely mangled sexually and thus would be 'neutered'. By your logic, wouldn't all sterile people be 'neuters'? Sure, they look male or female, but they're not, they can't do what a 'true' male or female can (reproduce).
Whereas. There are people with male sexes who are female gendered. People who are female in body and male in mind. People who are male or female in body and genderless in mind. And every single possible combination you can think of.
Intersex is the word for people who do not simply want to change their bodies from the sex they were born into, to the other.
Here's an exercise for cisgendered folks (your social gender matches your biological sex). If you're alone, say out loud what your gender is: "I am Male" or "I am Female" respectively. How does that feel? Probably like nothing; it's true. Now say the opposite: "I am Female" or "I am Male". Did that feel different? Probably felt a bit silly, because to you it isn't true.
Heh, I tried this. I said aloud "I am male" and then "I am female." The latter made me feel like a fool while I was fully comfortable with the former. It's odd, to me, because I am a woman and I am fine with that.