I think it's because they attach so much importance to being clever that their cleverness becomes a weakness.
Imagine you were always top of the class at school. You weren't the sporty one, you weren't the cool one, you were the brainy one. Your superpower was understanding things that were too complicated for most other people to understand. And then you left school and went to university and continued to collect qualifications until you eventually entered your chosen field and excelled at it.
Let's say your chosen field is law, you have a PhD in constitutional law, you became a barrister and then a silk and are now a judge.
Being clever is your identity.
And you frequently come into contact with people, often members of the public with no expertise in your field, who nonetheless have strong opinions about things like the death penalty, or human rights, or jury trials, and are quite happy to publicly disagree with you and say you are wrong.
You find it easy to dismiss these people as thickos who have no idea what they are talking about. If only they were clever and educated like you, they would have the same opinions as you.
Then along comes a doctor who prescribes puberty blockers to children, or someone like Sally Hines with her PhD in gender studies.
And they confidently inform you that anyone who thinks that a man is someone with a penis and a woman is someone with a vagina is just a thicko, who doesn't understand the difference between sex and gender.
You don't understand the difference between sex and gender either, but you don't want to admit it, because that would make you a thicko and there is nothing worse than being a thicko.
And you're certainly not going to say, "Well that sounds like total horse crap to me, Sally, and just because you have a PhD doesn't mean you're not a complete fricking idiot."
Because you have a lot of respect for qualifications, you attach an enormous amount of value to your own, and you have a PhD yourself. So obviously you are personally invested in it not being possible for someone with a PhD to be a complete fricking idiot, because that would mean that you also might be a complete fricking idiot, which is unthinkable.
And, you know, you're a legal expert. You're not a doctor or a professor of gender studies. These are Helen Webberley and Sally Hines' fields of expertise, not yours. And if they have that expertise and they are telling you that humans can change sex and that woman is just an identity, who are you to argue? You don't want to be like those ignorant fools who think their opinions about constitutional law are as valid as yours.
And you can see that, on the whole, you have people with degrees and titles and impressive sounding jobs saying that trans women are women, whereas it tends to be people who are a bit, well, Brexity, who say otherwise. OK there are a couple of outliers like JK Rowling, but at the end of the day she just wrote some books for children about wizards, so she's not really an expert, is she?
You know which side you want to be on. Even if you don't fully understand it, you're happy to defer to the expertise of doctors who have dedicated their careers to gender affirming care and academics who specialise in gender studies.
Because they must be right...right?