Because there is no species representation, meaning they are no social constructs. You cannot change your species, just like you cannot change your sex. Transsexual people might take medications or do surgeries that change their bodies physically, somewhat changing part of their sexual expression, but they can never change their entire sex, which is defined by many things including their genes.

Gender however, which is not tied to biological markers but social rules, can change. A hundred years ago women couldn’t wear trousers because wearing them would make them men and people found that unnatural. Some women wore them and anyway, some might have called them transgender at the time. Then gender stereotypes changed and it became socially accepted for women to wear trousers, and a woman wearing them isn’t seen as masculine nowadays.