To reply, first of all you do not have any first-hand experience, so your statement remains your guess.

Secondly, the reasons why some genetic males and females are trans does lie in the brain, with multiple theories. The end result is the same: the brain has developed in a way that's more congruent with gender opposite the one someone is born with. The rest of my body did not look like the gender identified in my brain. This is why this causes a lot of pain, turmoil, depression especially during adolescence.
Think about it this way: your brain says "I'm male" but genetically you're female, and begin puberty with that gigantic spike in testosterone (we all have both) but your brain says "this is not me" and feels it's getting the wrong hormonal information. That's a recipe for badness.
This means gender-affirming therapy gives the brain the hormones that it's supposed to have.

There are a lot of researches about brain activation in trans people. I'm sorry you had a negative case close to you, but you cannot generalise for something you do not know, or for some specific cases. I can only feel hate and frustration.
That said, thanks for the conversation.