Do you really not realise?
It's because the fantasy that some male people are so interchangeable with female people that they can be added into formerly single sex provisions (physical,.social and legal) without any consideration, indeed with the deliberate blocking of any such consideration under the label "transphobic", of what needs to change if these provisions are now mixed sex or of the impact on the original female users, is being leveraged to demand these things happen in reality, justified only by the meaningless catechism "trans women are women" with no substance behind what this word "woman" now means other than "ticket to women's provisions".
If the fantasy wasn't being weaponised to the detriment of female people there would be no need to keep reminding people that it has no concrete reality underpinning it.
Indeed, if the TRA position was "trans women are men who have an emotional need to live as women", acknowledging from day one they are not women (in the original sex based meaning), we would be expending all.this energy on finding all the ways we are similar and can share voices and resources.
But as long as the TRA position is "trans women are women", women (in the original sex based meaning) have to keep highlighting the ways we are different because otherwise this fantasy will subsume us and we will lose our own rights, our political voice and identity.
To go back to your analogy with religion, as long as religious tolerance requires me to do no more than accept others' right to live by the rules of a god or gods I do not believe in, and that I make external deference to those rules if I am in their religious spaces, I would have no reason to insist that their beliefs are a fantasy and indeed it would be rude, even hateful, to do so.
However, if a religious group was gaining political and social power to claim the moral right to require me to profess their beliefs and live by their rules in formerly secular spaces, of course I must refuse and call out the lack of any concrete reality behind the beliefs that would justify such a demand.