I personally think it's because a lot of people, to a degree, are still stuck in binary thinking.

Sure, they might acknowledge that gender is a spectrum, and people can identify anywhere on that spectrum, but they can often still perceive the underlying traits often associated to that gender as inherently tied to it.

So while someone might accept that trans girls exist, and are, in fact, girls, they might also carry with them the assumption that wanting your gender to be that of a girl means you also naturally want your body to replicate the stereotypical form of one.

It's sort of like a secondary wall that they have to break down. The first was regarding how gender itself is tied to identity, not solely to biological characteristics, but they've yet to break down the second of assuming that because you identify as something, you also want everything stereotypically considered to be a part of it.

I'm no expert, nor am I trans myself, so take this with a large grain of salt, but considering years ago, I once had similar inclinations before getting closer with various trans people I now know personally, this is the best explanation I can come up with.