To me it seems like the important question is:
Why wouldn’t one do something that makes others feel valid/happy/comfortable for so little effort?
It’s easy to respect name and pronoun preferences and admit when mistakes are made. One needn’t to dive into the full nuance and complexity of trans experience to understand that.
I think I can see the point of confusion- the reason for illustrating physical sex as a spectrum is that it’s easier to lay out the concept of gender identity as an analogue.
It’s also possible that gender identity is structural but brains are complicated and linking mechanisms to behavior is hairy.