Because the GRA only affects trans people
Having a fiction written into law, however well intentioned and I'd question even that tbh, to facilitate pseudo realities absolutely does affect other people.
Wrt the GRA this was known at the time the law was being passed but concerns were hand waved away because who cares if it affects women and girls negatively and it was all done on the hush hush QT so Joe Public weren't made aware of the full implications.
We know now, and it can't be whitewashed away no matter how much some would try.
Self ID means any man can change his legal sex without even having to claim a trans identity, with all that entails.
Coming to FWR to piss down our backs and tell us it's raining isn't going to cut it I'm afraid.