Because there's no physical marker or indicator for trans identification later in life. If there were we'd not really be having much debate over the issue and we'd probably have worked out as a society what to do about it similar to how we worked out how to deal with intersex conditions.

Logically it is a mental health condition. Beyond that, we don't really understand much. I think there are perhaps five different complexes that result in trans ideation. There may actually be more. It's hard to say. Blanchild did good research on some of this. But he only identified two categories. I think there are more.

TRA don't like it when you say that being trans is due to a mental condition because it automatically makes their arguments fairly weak if you can simply write them off as insane. While I believe you can, that is a rather boring argument, so I will assume good faith and sanity on the part of the person I'm having a discussion with.

There is some subset of reasonably sane trans people I can get along with well enough, who seem to understand that they aren't actually the opposite sex. Don't care if they pass or not. And just do it because they want to. Don't try to impose themselves in places where they aren't welcome. And don't go around trying to convert kids. Naturally these people are also quite boring and so they don't get much attention.

Calling anything xyz phobic is simply a shitty debate tactic used to derail a discussion. The best way to deal with it is to either ask why, and let the person get frustrated when they can't explain it without tautologies or emotional appeals, or to simply accept the label as if it doesn't bother you the tactic fails miserably.

As such if people call me a racist, a Nazi , a transphobe, homophobe , whatever, alright. I'm a communist conservative ultranationalist Christisn athiest anarcho-monarchist racist universalist homophobic faggot. Now let's talk about how it's physically impossible for anyone who has XY chromosomes to have babies. Please present your counter argument to this fact so we can discuss it.