Because they are different. No transwoman can give birth or have a period. None of them has a womb. None of them are genetically female. Most of them have all the advantages of a man like higher bone density, more upper body strength.
Why are you so afraid of different groups of people being physically different? As long as you treat all groups as humans, it shouldn't be a problem. Denying basic biological facts is not a path to truth.
The distinction is supposed to be that "biological facts" no longer include gender. Gender is now fluid psychological term, not a biological one. In other words, you said "advantages of a man" instead of "advantages of a genetic male", which is now wrong.
But apparently I'm incorrect in thinking that you're not supposed to discuss "trans women" as a group, which is still confusing to me.
Sure, gender is different from sex. The point stands. There are major biological differences between females and trans women. And there's nothing wrong with that. It's just a part of life. Pretending everyone is equal is just lying to yourself.
But of course, people should be treated equally (in most cases).
Why are you so afraid of different groups of people being physically different? As long as you treat all groups as humans, it shouldn't be a problem. Denying basic biological facts is not a path to truth.