it is still treated as a package deal even by transgender people: you perform either the standard male package or the standard female package

Trans people engage in more variation than that, actually.

modify their bodies so as to more closely resemble, at least outwardly, the biology of their desired sex

Not all have the same motivations. It depends on what you want your body to do. Some trans people care that others will treat them according to the image they present: it may be difficult for others to think of you as the man you feel you are if you lack the conventional signals (beard, voice, flat chest, etc.). For other trans people, erotic possibilities are the deciding factor: your body parts determine which sex acts are possible for you. Still other trans people may find that it's more an issue of overall self-image. And some trans people may simply not mind any of these factors and decide not to have any surgery at all.

For both issues, a more general reply is that acknowledging the variety of possibilities of human sexual experience involves acknowledging further variation at the individual level.