I guess because they feel that they feel like they born to be their preferred gender. The idea that there will be a treatment that will make me feel like a gender I'm not sounds pretty f-cking terrible to me.

Try this experiment on yourself and think that there is a treatment to make you think you are woman/man. How's it make you feel? And together with that, reflect on what exactly make you be a man/woman - your body or your psyche.

There is a lot parallels between gender dysphoria and BIID, but GD is bit more than just 'this is not my body'.