Because the Church does not have a stance on whether transsexualism is legitimate or not. Transsexualism does not state that one can change or choose their gender. Transsexualism is a medical disorder where one develops the wrong genitalia in-utero. Surgery is merely to correct the in-utero error using tissue the body would normally have initially constructed their genitalia based on their gender.
If you wish to believe that gender is entirely dependent on the external appearance of one’s genitalia (or chromosomes or internal organ composition or whatever you want to use as the basis of gender), you are more than free to. But the Church has never definitively stated that the shape of one’s genitalia (or chromosomes or internal organ composition) is the defining factor in one’s gender. In fact, the fact that gender is shown to be present before and during conception and after death in the Theology of the Body should very clearly indicate that there is an infallible definition of one’s gender. It cannot be altered or chosen, period. Even if one is to go into a child’s DNA in-utero and alter their genitalia production, it does not change their gender, even though they would develop reproductive organs of the gender opposite their gender.
I believe transsexualism is legitimate because the experiences of transsexuals that I have known in the LGBT community (having a clear identity of their gender, having it be present unceasing for their entire life, having positive medical responses to hormonal and surgical treatment) are completely in line with Catholic teaching on gender. But every Catholic is free to believe however on this issue. There is no Church teaching on the determination of gender, period. Only that it cannot be chosen or changed.