Why is the stuff going on in my head the only thing that matters in my body
Because it has nothing to do with your body. It has to do with identity, which has everything to do with how you fit into society, who you interact with and how everyone else sees you.
Transgender people aren't delusional. Nobody's stupid enough to just say "hey i'm now a woman when i was a man before." they know they are biologically male, but that doesn't actually matter. They choose to identify as something else because that's their choice, and other people should respect that.
You have the choice not to respect other people's identity, but then you're making the conscious choice to be rude.