Because the right to call yourself "trans woman" has been widened to the point of including people who didn't undergo any gender reassignment surgery or hormone therapy. And yes, I don't think that those "trans women" are, or should be considered, women. Actually, not even "trans".
At the end of the day, it's their decision what to do with their body, not yours. All that is asked of you is to respect their decisions and treat them with the same amount of respect as you'd treat any other non-trans individual.
If we meet in public and you say "Hi, my name is Michael" I can only assume that that's objective fact. If I then say "You know, you don't really seem like a Michael. I think you're more of a Denise based on what I've seen." You would be right to take offense for disregarding your own right to self expression based on my own interpretation of your person from the limited information gathered in a first impression.
This is a very similar thing, except by the time someone is out as trans you can best believe they've spent years agonizing about whether it's even a good idea to do so knowing they'll face this kind of a conversation every time the topic comes up around people who they aren't close with / are not sympathetic.
An attribute that can change based on someone's feelings is definitely not objective. And all of the genders that are neither male nor female were invented in the last few decades, so membership in one can't be factual. Likewise, membership in a construct called "gender" which is divorced from biological sex was also invented within the last century, so that can't be said to be factual either.
Objective [1]:
1. Of or relating to a material object, actual existence or reality.
2. Not influenced by the emotions or prejudices.
3. Based on observed facts; without subjective assessment.
Fact [2]:
1. Something actual as opposed to invented.
2. Something which is real.
Furthermore, from a scientific standpoint, there's not really any such thing as a clean binary division between male and female. Chromosomes get messy, and even absent issues with the X and Y chromosomes themselves [2] there's some fun stuff with the SRY gene [3] and hormonal receptors in utero that can affect gender identity and presentation [4].
[1] - https://en.wikipedia.org/wiki/Transgender_history [2] - https://pubmed.ncbi.nlm.nih.gov/9396296/ [3] - https://pubmed.ncbi.nlm.nih.gov/20184645/ [4] - https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6235900/
Actual intersex conditions occur at a rate of approximately 0.018%. [1] This is a pathology, not a third category.
You say this very lightly but it's obvious to me that you haven't really considered the consequences.
For example, you say "people who didn't undergo any gender reassignment surgery or hormone therapy". But you don't seem to consider that those are medical interventions. I think if you thought of them in that light, you would be a little more thoughtful about demanding that people undergo medical interventions for any reason, no less as the price of being recognised as their claimed, or lived gender.
Such requirements are common. In some countries in Europe until very recently transwomen were not allowed to change their papers to identify them as women and to declare the female names they used everyday unless they could demonstrate that they had been rendered surgically sterile, by castration. Germany was one such case.
I wonder also if you have a slightly romantic idea of surgery and hormone therapy. The truth is that surgery doesn't magically transform a man into a woman, no matter what some surgeons want us to believe. Some transwomen will always look like men, no matter how much they cut off and throw away. Others, will look like women without having taken any hormones in their entire lives. I know that's hard to believe, but it's how it is. The human species is only very lightly sexually dimorphic and some males can pass for women, and some females for men, with very little intervention. In the case of transwomen, for many it is enough to remove their beard (by electrolysis or photolysis) and take care of their hair, to be immediately identified as women by everyone that sees them. On the other side, history is full of females who cut their hair short, wore pants and lived the rest of their lives as men.
But, I know the above is hard to believe so OK. Just please try to keep an open mind and remember that you don't automatically know everything there is to know about transwomen (and transmen).