Muscle mass and density, twitch muscles, skeletal proportions, bone density, lung capacity, different hormone levels including adrenaline...
There are plenty of men who are weaker than the average woman, and women who are stronger than the average man, but there tends to be marked differences in general. Men and women are built somewhat differently, and it's not just for the sake of women looking prettier. And that includes characteristics that cannot be altered by surgery or hormone injections.
It's not exactly an accident that men who are altered to appear more like women have a greater tendency to outdo the competition when they're biological women in sports. They tend to have it unfair advantage, and that's the very reason why we have sexually segregated sports to begin with. It defies the purpose when somebody grows their hair long and changes their pronouns or something and wants to get in a swim meet or boxing match against biologically born women.