Because there's a difference between discrimination and racism (or sexism, heterosexism, ableism, etc.). In other words, the same power dynamics that create
white/male/heterosexual privilege have created a hierarchy of prejudice so that discrimination against a privileged group is not the same as discrimination against a non-privileged group.
White people can be and are racially discriminated against. But there's not an entire cultural, ideological, and structural system of power in place that condone and support this discrimination. It is still discrimination, it is still wrong. But it's not the same thing as racism, which is commonly defined and understood as discrimination + power (institutional, cultural, ideological, structural).
On a similar note, men can be and are discriminated against on the basis of their gender. But it's not the same thing as sexism, ie, gender discrimination + power.
I don't think that anyone would dispute the fact that discrimination sucks and is wrong. But glossing over the inequity of the two discrimination helps keep the inequity in play and the privilege of some groups over others intact.
Creating safe-spaces for oppressed and/or vulnerable groups is not discrimination. Those safe-spaces aren't created on the ideological belief that the privileged excluded group (white, men, hets, etc.) is inferior or less deserving of certain rights or privileges. Women-only gyms for instance are created not with the purpose of denying men a space to work out, but rather with the purpose of providing women with a space where they can feel safe to work out without having to deal with unwanted attention. Creating those women-only gyms doesn't deny men their access to most otherwise mixed gyms.