Sex doesn’t equal gender because they are literally two different things. It’s like asking why hamburgers aren’t hot dogs.
But within the context of this conversation, you are taking a hot dog, grinding it up and calling it a hamburger. On the one hand, does it really matter? On the other hand, there is a reason that words have meaning. Meanings can also change.
We can’t get rid of gender even if we wanted to because it’s not a consciously created construct. More importantly what it means to be male or female is different in every culture so if we said sex and gender were the same thing that means we would have to deny reality.
What we can’t get rid of is biology. Gender is a construct, yet most gender norms also reflect certain biological realities...for instance, marketing baby dolls to the maternal instinct of little girls.
I call it flat earthing biology because most liberals criticize religious conservatives for ignoring science, yet from a pure scientific and evolutionary standpoint, sex is a fact based classification system developed by evolution to ensure the survival of the species, where gender is about “the feels”.
If our species were to go extinct, and millions of years from now, an alien species were to study our remains, would gender even factor into how they classify humans?
Gender is simply another way for people to communicate their role in society. Respect shouldn’t require a periodic table of classifications.
And having said all that, I think its ridiculous for girl athletes with the biological advantages of males to compete against biological females, even though I also acknowledge that religious conservatives are exploiting this inequity.