I think it’s a combination of two things: one, they’re straight men (specifically “transbians” not all transwomen). They don’t want to date other transwomen because they know that transwomen are men even if they can’t acknowledge it. I think the reason they don’t seem to go after bisexuals or “pansexuals” (not that being bi would mean that someone is open to dating a transwoman) is because of reason number 2: validation. If they can get a lesbian to date them, that’s kind of maximum validation. As another poster said, they don’t care about how the women they’re going after feel, it’s all about them wanting sex/love/companionship and validation.