For some, if they admit that gender isn't fixed, they'd have to admit they waste so much of their live not transitioning. Sometimes its easier to double down when you're wrong for so long.

For some men, its probably mostly about sexism. They have so little going for them personally that they latch on to things like "being manly" (or "white pride") to compensate for their insecurities. If "men" choose to be women, then that's seen as an attack on the superiority of being a man and to maintain the exclusivity, you also can't allow "women" to become men but at least "women" wanting to be men supports the belief in superiority of men. One reason why trans women are demonized more than trans men by some transphobes.

For some women, its probably similar to how some minorities because even more prejudiced against some other minority that is seen as even "worse" than them to join with the majority. Like racism against black people by Irish when they weren't considered white or homophobia among racial minorities.

For others, its just parroting their bubble.

Some, just because they expect to be able to get away with it.