Because gender presentation (outward expression) is fundamentally different from gender identity (inward sense of self). "Transgenderism," by the way, isn't a thing. The only people who talk about "transgenderism" are transphobic bigots — excepting people who are uninformed and use the term by mistake.

Transgender humans are people, not isms or ideologies. If you want to ask why they identity as transgender, ask them. They might be willing to share.

Asking Natalie, a cisgender woman, won't get you anywhere. She's not the right person to ask.

Some trans people might not want to answer a question the way you asked it, which is quite hostile and offensive.

What it boils down to, really, is that what you think as a cisgender man is of no bearing. Trans people can, do, and will identify as they please — regardless of your opinion.

Because trans people belong to themselves, not to you. You don't matter.