In this case, I'd argue it wouldn't be specific enough. When the game contains a multitude of races and widely-available gene-altering medicines, cybernetics, androids, etc. all of which can have differing levels of appeal for the game's audience, differentiating between a man who looks like a lady, or a man who identifies as a woman and altered their body to look like a lady, or someone who was born with both genders and looks femine, or a woman who got themselves some man parts, or a lady who looks like a man...
...This game is weird, is what I'm saying. At least right now we have some semblance of order. It's already getting hard to navigate the character lists on the wiki with the ~300 characters it lists at the moment...
EDIT: or, you know, instead of writing all that, I could've said "because their writers want them to be trans and not herm/futa" and call it a day. Futa/herm only specifies that they have male and female sex organs (last I checked), while "transgendered" has more going on that can inform the character's history, psychology and motivations, in addition to their physiology.
...I mean, it's not like there's just two guys who are in charge of writing all the dick-wielders. If you mean to say that there's not much new to add to the concept in game, in which case, I can't really comment much on that part, as I'm not a writer.