Because they want to degrade someone. A crossdressing boy is their kink because they think it's degrading. They also only see it in an erotic context. They don't see a boy dressed as a girl as a fellow person or member of society, just an object of desire. They see a slab of meat and an outfit.
Whereas a trans girl with an explicit, stated identity is someone asking for respect. She's someone who announces her humanity and doesn't primarily exist in an erotic context. She's a full human being they have to reckon with. A trans woman's gender is a normal aspect of her identity and goes beyond being a kink or a type of clothes.
That's my guess from reading how they talk in forums about this stuff. They're also misogynists so throw that in the pile too.
Boy dressed as girl is hetero because straight boys can have a little bussy as a treat, as long as it's not argr (NB: getting pegged is not argr if the manlyboy still gets to treat the femboy as a sex object).
Trans girl is gay and you're gay for liking her because gay is when the sussus amogus is subversive to the modes of patriarchal oppression like some kind of soyboy loser (this includes respecting trans people's identities) (patriarchy is predicated on gender essentialism)
Argr is Old Norse for "unmanly". The Norsemen would often have male-on-male sex, where the dominant partner would be seen as manly and the submissive as effeminate, or argr. So if you were accused of being argr, i.e. of being passive in sexual relations with another man, you were literally left with the choice of either killing the accuser, or becoming an outlaw due to being too submissive to take another man's life.