Because literally the entire episode is basically "You will never be a real woman, no matter what". Perpetuating the that trans people getting gender affirming care don't know what they're doing and are just mutilating their bodies. There's literally a line where Garrison is talking to their doctor and says something along the lines of "Wait, so I'm basically just a dude with a fucked up dick?" and the doctor responding in the affirmative.

I genuinely really like a lot of stuff from Trey and Matt, but that's one of a couple times where it felt far less like "poking fun" and was intentionally portraying trans people in a harmful way, in a way that an episode like "The Cissie" didn't do nearly as aggressively.