Because of the parallel. Prejudice follows the same patterns. Always has. And it's useful to take someone's position on one controversial issue and extrapolate that to another that is much more black and white - tends to focus the mind when people realise that their arguments against transgender rights are exactly the same arguments that racists make against racial equality...
Racial inequality was reduced when segregation ended. The group with less power, black people, wanted segregation to end, because the system was put in place and maintained by those with more power.
Women are the group with less power when we are discussing sex segregation. Women do not generally want sex segregation to end. They argue that they are at a physical disadvantage to men (regardless of the claimed gender identity of those men).
So are you saying that women arguing for the retention of sex segregation are like white people arguing to retain racial segregation? Because it seems to me you are ignoring/reversing the relative power of the two groups in question.
And you may be arguing for an end to sex segregation, but most transactivists are arguing for men to be able to use women’s spaces at will.
The equivalent in terms of apartheid would be for white people to claim they were black and demand access to the spaces that black people used, despite the black people saying that caused them distress.
Can you explain the parallels please? The situations and arguments appear wholly different.