Because a shitload of people are transphobic and they genuinely don't understand it in the slightest, so instead, they base their views on the most biased and extreme examples they can find.
It is the exact same kinds of things people did (and often still do) to homosexuals. Like when people think of a gay man, what do they think of? Probably someone effeminate with a lisp and a FABOULOUS fashion sense. It is a stereotype. And the stereotypes used to be much worse. What does the average person think of when they think of a trans person? Probably a man in drag (which isn't even the same as being trans).
They said gays were pedophiles, mentally ill, degenerates, and perverts. They said that they would corrupt children and lead to society rotting morally. They said accepting them would destroy the natural concepts of sexuality. They said that being gay is a choice and that all humans are straight. That being gay as a concept didn't actually exist, it was a lifestyle. They said "I don't hate gay people, but I don't think they shouldn't shove it in my face" or something similar. If we let them get married then who knows what people will get married with next! Trees???
Now, what do they say about trans people? They are pedophiles, mentally ill, degenerates, and perverts. They corrupt children and rot society. Accepting that being trans exists destroys the idea that there are only two genders. Being trans is a lifestyle, there are only two genders. "I don't hate trans people, but…". "If people can 'choose' their gender then people will identify as attack helicopters!".
It is the same old shit with a different target. Except unlike being gay, being trans is a genuinely hard concept for people who aren't trans to understand. So they try to understand it in completely inaccurate ways, such as "choosing to be trans" or "don't they know they have X genitals??" or its when a boy likes feminine things or vice-versa.
People genuinely hate or are creeped out by transpeople. So when they see some idiot only like posting something stupid that confirms their pre-existing biases, they latch onto it and use it as ammo against trans people despite the fact they are in the extreme minority. The post above exemplifies this.
It is not different than seeing a black criminal on TV and claiming that it is clearly an exemplification of black people.
The worst part to me is that these people try to pretend that they are not bigots. At least own your shit and stop lying to yourself and others. Like a racist who claims they are not racist but constantly rambles about all the things they hate about some minority group.