In all seriousness, why is being transgender not considered a mental illness? Also, why are people transgender to begin with?

Hi so I want to start off by saying I don't have anything against transgender people. It's their body and how they choose to express themselves/ identify is on them. Should someone come up to me and explain they want to be called by certain pronouns or whatever I have no problem with that. The thing is I personally don't understand why it's not just classified as a mental illness? I feel like for someone to think that they are a different gender than what they were born would be some sort of chemical imbalance in the brain or something?

On a related note, I don't understand why people feel the need to change? From talking with some people usually what I hear as reasons to be transgender is because they feel like a guy/ girl/ different gender than they currently are. I guess it's just hard for me to wrap my head around that concept because to me if they feel like doing guy things but they're female or vice versa then just like, do those things? What's considered manly vs girly is such a societal thing I don't get why it matters? I guess it's i just really don't see a purpose for wanting to change your body that much.

So yeah I guess I just don't understand why being transgender exists and I'm just curious for any responses that can help me understand better