Because actual Trans people don't exist, they're a lie, and let me tell you why.
When you body is formed, right in your mothers womb, your Sex/Gender is decided by nature alone. Yes, Sex and Gender are synonymous to each other, it has nothing to do with social constructs or anything.
The Male and Female body also have way more differences than just genitals. Brain structure, bone structure, skin structure, everythings different and most of these can not be changed in any way. So, even if you are post-operations-trans, you're still the Gender that you were born with - except that now you have torn up genitals.
And heres the thing about Genderfluids - its a lie. You WANT to be treated as a female, even if you're a guy, i get that. but telling people that you ARE a female, just because you want to be treated as one is the very definition of a LIE.
lets say i love eating meat, and i love banging vegan chicks, so i tell them i'm a vegan as well, so that i get treated like a vegan, without actually being one. Thats a lie too, right? And this analogy is really important, i want you to let this sink in.