It all just boils down to them knowing that any objective standard would invalidate a significant chunk of their members. If you create an objective standard, then guess what? Some people won't fit that standard, and suddenly you will have definitive proof that some trans people (probably most of them if we're being honest) are faking it for sympathy/diversity/malicious intent. This is why they cannot come up with a logical answer for the "what is a woman?" question. To define the word woman would require an objective standard which we could compare against all self-identified trans people, and you know some of them will not make the cut. Then you will know those ones are faking it, and then suddenly, all those evil transphobe arguments - rapists lie to gain access to women's spaces, you just think you're trans because you went through trauma, you're just faking it for ass-pats, etc. - have real validity and weight to them. Then, suddenly, every tranny will be subject to scrutiny, and the jig is up. No more perving in the women's locker room for you!