Pedro Almodóvar's 'The Skin I Live In' (2011) is what i always think about when i consider why trans people need trans therapists.
If you remove a replace a trans person in a story of transition with a cis person, the tale becomes one of intense body horror. The force that pushes on trans people creating the natural need to bring ourselves into alignment with our gender identity, pushes equally on cis people to strengthen and defend their gender identity. The idea of having that stripped from them is abhorrent, as Almodóvar demonstrates.
So i, personally, will never be able to believe that chasm of understanding is fully bridgeable in understanding in a way that a cis therapist can actually find connection with a trans client. You can translate, you can find metaphors but the fundamental core need that paints all aspects of a trans persons life is completely alien and wrong to a cis person's being.