Well, I'm not able to posit an answer to this as to why trans people don't consider it valid, but I've seen discussions about it.

What I can point to is the cis side of things. Ignoring the jerks that are coming from bigotry, berated that's the answer for them: bigotry.

But I know some people that are supporters of trans rights, but still hold onto that view, that until someone, particularly mtf people, has bottom surgery, they haven't transitioned, or at least not fully.

Now, it took me a bit to wrap my head around it, why someone that would want to transition at all wouldn't want all possible methods used. And it comes down to the binary. People in general still haven't grasped that not everyone is binary in the first place, much less that transition is not necessarily about changing the body.

It's certainly a big thing, it's important, but making that mental leap from "oh, they want their body to be right, and that means being as direct a match to the organs and body shapes of the confirmed gender as possible" to "oh, everyone can have different needs regarding any given part of their body to be transitioned for themselves"

See, a lot of the fight about trans rights revolves around genitals. A lot of what people that are binary build their idea of gender around as they learn and age is the sexual characteristics, with the primary characteristics of genitals being the most defining. In other words, me being a cis man, my gender identity is very much tied to my genitals as the most relevant factor in my body matching my inner self. I've moved past it being the only factor (though that took time), but my junk is part and parcel of the physical aspects that make my body a man's body (for my body, not others')

When the tie between gender and sex and identity are first developed, it's inside the self. So the closer one hews to the binary, the more import ends up being placed on genitals because of being at the binary points. "I am a boy/man, and I have a penis, therefore men have a penis". We all start with our own viewpoint.

And, for some trans people, genitals are a primary factor. But they don't have to be, and it takes a combination of exposure to the possibilities as well as willingness to change one's mind to truly grasp that.

You know the most significant thing that helped me get that transition confirmation isn't purely about surgery? It was talking to trans men. The bottom surgery options aren't as sought after among the trans men I've known. Hormonal effects have turned out to be perfect for them. It confused me at first, what with my gender identity being significantly tied to my genitals looking and working in a distinctly cis way. But it eventually broke through that you don't need a cis penis or testicles to be a man.

And that is what directly led to internalizing that the notion surgery for trans women isn't necessary either. If I lose my penis, I don't become a woman. Recognizing that fact flipped my head. It means that, just like phalloplasty not being necessary to become a man; you don't have to have vaginoplasty to be a woman.

A woman that happens to have a penis is still a woman.

But it takes knowing enough trans people, having enough conversations about it, to break out of the binary way of thinking even for trans people that are binary.

To refocus, I think that the reason people think you have to have bottom surgery is that they've not had the right combination of events to make them break out of binary thinking as the default. Now, the ones that refuse to accept the fact of it not being the defining characteristic, that's as much about humans being stubborn and sometimes arrogant about our own thoughts and opinions being the right ones. Most of us don't want to change our minds at all, about anything.

But I suspect that, given enough time and exposure to the concepts, anyone that's otherwise on the path of understanding trans people will get there eventually. It's only the ones that aren't trying to understand that won't get there.