It's because we're not allowed to define ourselves as a cohesive group by our biology. To do so is supposedly trans exclusionary.
But from a completely neutral standpoint, why is defining by biology exclusionary, yet defining by gender isn't? I feel less of a woman and excluded when women are defined by gender norms (I mostly wear trousers & wear makeup about 4x a year on a night out), but that's ok I guess as I'm just a woman. But I just told you how I'm less of a woman... just shows how stupidly circular this discourse is. It comes back to needing biology as an anchor point to understand WHY women are oppressed in the first place.
Also, woman are happy to include TIFs in the biological definition; male trans advocates wish to exclude TIMs, so surely they are equally as trans exclusionary?
The faultline comes down to subjective gender identity vs objective material reality. It goes far beyond feminism even, into epistemological philosophy - how do we know what we know and what is real. I've had arguments with twatters in the past few days who believe that sexual dimorphism is a social construct... like the whole of (sexual) biology only exists because human minds can rationalise and categorise it. Like it doesn't exist empirically outside of human observation 
Reposting this extract from 1984 to illustrate
"Only the disciplined mind can see reality, Winston. You believe that reality is something objective, external, existing in its own right. You also believe that the nature of reality is self-evident. When you delude yourself into thinking that you see something, you assume that everyone else sees the same thing as you. But I tell you, Winston, that reality is not external. Reality exists in the human mind, and nowhere else."