Because most people don't differentiate between 'male' and 'man'.
Why?
No, it's quite simple. Just respect a person's self-identity and don't try to weasel around it. Unless you are a doctor, a person's biological makeup shouldn't be any of your business.
Language is constantly changing, and many positions don't make internal sense.
I know what you mean though. I once dated a lady who called herself Josephine. Imagine my horror to find out that wasn't her real name. Turns out she just used it because she didn't like her real one. She didn't even bother to change her actual
sex name, just went around telling people that she was a
woman Josephine now.