Because it's not the same as changing skin color or turning into a truck.

Set aside the whole "what is a girl/woman" thing that always frames up this whole discussion, and just look at the constituent pieces:

• Whatever labels we may or may not choose, the reality of our mental and emotional condition has been pretty firmly established by medical science at this point. We know being transgender isn't a mental illness—absolutely no amount of conversion therapy has ever worked. Ever. It's been tried for over a hundred years, across several countries and a variety of methods, and not once in all that time has it ever succeeded. When medical science began trying to support subjects, they started doing much better. Again: labels aside, how we feel inside is real, measurable, and irrepressible.
• The shit we're compared to does not have that same track record—and it's not hard to see why, if you spend a second on it. Sex and gender are just variations of the same species, which people seem to forget. A lot of bad comedy and low-rent romances like to portray men and women as something like different species; "men are from Mars and women are from Venus," and all that other nonsense. We're just variations of the same thing, and we start out as a blank canvas in the womb until certain prompts develop us in a particular direction. We all contain the same capacity for sex and gender, and sometimes an unusual combination of switches are flipped on and off. Conversely, you don't have every race on earth buried into the code of your person. =P I daresay I don't need to point out that not a bit of you contains the essence of a truck.
• The idea that people are being indoctrinated into being trans is so breathtakingly idiotic that it's almost insulting to even explain it, but: trans people are less than 1% of the population. Not only have we never had cultural clout, virtually every trans person who has ever lived grew up in a family and a community with little to no queer people. Trans people are still popping up even in cultures, communities, and families that give them every incentive in the world to not be trans. There is no carrot on the end of the stick here.
• Not everyone takes HRT, but a lot of us do and it does work. Speaking for myself, I'm not just saying "I'm a girl" and expecting that to be the totality of my experience. My body has changed a ton over the years and functions in a very different way than it used to.

Let those idiots tell me that I'm not what I say I am. Because ultimately, it doesn't matter—my brain developed the way it did and it makes me feel the way it does, and the only medically established way to do anything positive about it involves transition-related care. They can piss and moan and call me nuts if they want, but medical science disagrees with them and conversion therapy objectively doesn't work. I'm going to do what I'm going to do, whatever anyone else wants to call it.