Trans question, sorry!

[brownjumper]

I know this subject has been done to death but I can't seem to find out the answer to this.

Why are trans people not happy with being trans people? They say "trans women" are "women" but if the first thing is different to the second then surely it is obvious they cannot be the same thing. I totally support their right to change gender etc, but how is the argument standing up that you can change biology?
But my main question is:
why are they not happy with being a separate "thing", a trans person. Why do they want to join themselves to the sex of the gender they want to be?

Just like the last sentence in this article:

www.economist.com/open-future/2018/07/03/gender-identity-needs-to-be-based-on-objective-evidence-rather-than-feelings

I am not female and I know that I cannot become female, but I can and do live in a way analogous to the way that women live. I make no claims I cannot justify and my life is better for it.