Another reason I think therapy transference is so sketchy is that the client can never really know the therapist. Them's the rules. The therapist must avoid excessive self-disclosure (i.e. be secretive), else it ain't therapy.

If the client can never really know the therapist, and the therapist pretends to be magically attuned and endlessly caring, the idealization might never dissolve. The client is stuck chasing the image of this figure who will end all suffering. And this figure ain't real. So the client is effectively chasing an apparition and furiously projecting all sorts of things.