It’s actually pretty simple...

If a character in a movie or TV show is written as a woman, a woman is automatically going to be cast for the role. Men won’t show up to audition for the role because the character is a woman. Directors aren’t considering a man to play the role because the character is a woman.

Trans women are women, not men in dresses. Stop putting male actors in dresses just because the character is a trans woman. It’s not about “the best actor for the role”, it’s about routinely portraying trans women as something that they aren’t: men in dresses.