This is like asking "why is a car not an airplane?" since the structure for both of them typically starts out as sheet metal, and asserting the question is more valid because you know someone who has a car and insists it's just an airplane without wings.

Actually, even that analogy doesn't go far enough, because someone with enough money to waste could take the materials from a car and make a shitty airplane out of them, while with sex reassignment surgery you can only make one sort of look like the other, but you can't replicate the functionality.

Edit: I just realized who wrote this article. I remember this clown. On Twitter he once tweeted something so obnoxious that's it's burned into my memory as an example of the mental gymnastics these people engage in to avoid acknowledging sex.