I think a lot of it is because of the narrative that a TW is a real, essential, somehow "true" woman, suffering even more than most women because of being trapped in the "wrong body" (much as this phrase is being questioned, I think it's still how a lot of well-meaning people see it). To think this you have to push aside thinking about a definition of women and instead just think that a TW is a woman just like yourself in some inner sense, who can be helped by being welcomed by you, and having any concerns or questions would be just mean.
A few years ago when it was taking off as a big, there were several BBC articles about children who were identifying as the opposite sex and transitioning at school etc - some really young. There was a narrative that e.g. male child didn't just "want to be a girl", they explained to their parents that they were a girl and the implication was that we all needed to grasp this important fact.
It was partly this that set me thinking about how a boy could possibly KNOW they were a girl and what these things really mean. And of course I think what the child really means is that they must be a girl because their feelings and tastes don't fit with what they are told a boy is meant to be, especially if adults are affirming being trans as a solution.
But if you just take "I'm really a girl" as the truth, it becomes this tenet of trans identity that cannot be questioned because the person themself knows best right? Even though when you think about it with a clear head, a TW literally cannot know how being a biological woman feels - as many TW themselves describe.