Because, he was born with all the male parts, but still claims to be female. Every DNA test known to man will tell you he is male, but he still claims to be female. Or in other, even more crazy examples, they claim to be inanimate objects.
If you did a DNA test the entire body would be female, except for the brain because it came from a male. But there is a big difference from literally combining two different people, and a person pumping hormones into themselves and then trying to say they're something that they aren't. Also, that example is like something out of a manga, I highly doubt that will ever really happen. 😅
What research? I'm not doubting that it's true, I seriously want to see it. Although I don't see how their brains could be closer to a male whose left arm is female (no seriously, that's not even the dumbest example.)
And again, we're talking a person pumping hormones in and out of their body repeatedly to pass themselves off as the opposite sex, not a person or people having a brain transplant. Now I'm no expert but logic would dictate one of the following would happen if it WERE possible.(I'll use a male brain and a female body for example)
1.) The male brain would cause the female body to have more testosterone.
2.) The female body would cause the male brain to have more estrogen.
3.) Nothing would happen chemically and it would literally be a man inside a woman's body.
4.) The two would reject each other do to their genetic differences. Honestly this seems the most likely going off of what I know about organ donations.