The same as a man cannot become a woman or a woman a man.
No, it really isn't.
A trans woman can identify as a trans woman because she is a trans woman.
It's not deceptive or delusional, it's honest. That's who she is.
You can debate whether trans women are women or not. That is certainly an interesting question.
But the idea that people just randomly decide that they're trans and they can just pick whatever identity they feel like is asinine and ass-backwards.
It's the same old attack helicopter bullshit that fundamentally misunderstands what identity means.
Yes, you can tailor some parts of your identity. Fashion or politics or other things you can adopt or abandon. I'm sure there are some "trans" people who are doing it as an aesthetic or political choice. Frankly I think a lot of non-binary people are doing just that.
But for most people there are some things that are core defining characteristics of who they are as a human being. And for most trans people that's what their gender identity is.
I would argue that many trans people know who they are even better than most "normal", because trans people have spent years and years wrestling with it, thinking about it, denying it, and finally accepting it.
No amount of therapy or other treatments have been successful in treating gender dysphoria.
But simply letting them be who they are is remarkably effective.