That is because it is disrespectful. On a whole other level.
Women (female people) have a right to our own stories and voices. The shit that happened and happens to us because of our bodies, good and bad, and the way our real lives are reframed and constrained by social stereotypes laid on our bodies, is something trans women will never experience.
Hundreds of thousands, maybe millions, of women over history have been blamed, sidelined, divorced or murdered because they did not produce the desired male heir. None of them were trans women.
Through history, women have been shunted into gender roles that give us status but also limit and constrain us. Trans women take these constraints and believe they are the definition of us.
If you really can't see why this matters, if you value female voices and experiences so little you are happy to have men stand in for us and think it's the same, I feel very sorry for you.