Most replies seem to just say "Because it's factually untrue" and while, yes, that is indeed the case, what's much more important is why these ideas are misogynistic and regressive.

First, we don't define a woman as anyone who "passes" as a woman, otherwise we'd just call butch or androgynous women men. We define them as female people. To define them according to how much they pass is to define women according to an artificial harmful patriarchal construct, rather than the reality of being female. Women do not become female through "passing"; they are born female and they remain female always (even skeletons can be sexed). Their female physiology doesn't go anywhere or change regardless of how they feel about it. A man might trick me into thinking he's a woman, but that still doesn't make him a woman any more than me thinking a butch woman is male makes her a man, and it certainly doesn't make him more of a woman than her (which plenty of QT people will actually argue).

Second, gendered socialisation is ingrained into people from their youngest most impressionable years, and this is impossible to avoid. The results of this are very apparent in the majority of trans people, especially since their ideology is not conductive to questioning gender roles on which they rely on for their identity. Transitioning does not erase this socialisation, hence why it's harmful to women to let men speak in the name of women and share their spaces (similar problems like with, say, male feminists). It is also harmful to tell girls who are saddled with very stressful freedom-limiting misogynistic and beauty expectations treating them like sex objects that they're trans if this cruel reality causes them fear and anxiety.

Third, trans people are extremely invested in and euphoric over performing a certain gender role, to the point of suicidal ideation if they can't do it. Plenty of them have fetishistic reasons for this revolving around emasculation and female inferiority. This has nothing in common with women who are women because they're female, nor men who are male because they're male. Women or men did not choose their sex, just as they didn't choose their shoe size or their eye colour. As such, even when male trans people pass and experience patriarchy for themselves, they tend to view it through euphoric "Imma real gurl now!" lens (which is why they often hold sexual harassment in fond memory as validating). Trans people in general tend to promote regressive ideologies for validation (things such as dismantling gendered spaces, gaslighting about gendered issues, brainsex theories, defining gender by gender stereotypes), and women are pushed into accepting this in order to validate them, both because women's rights always have to take second place due to what an integral ever-present facet of society their oppression is, and because women are socialised to be endlessly accommodating.