Because that's what you've been fed by patriarchy - that either men have to be "on top" or women do and there is no other choice. It's intentional defamation of the actual goals of feminism which have nothing whatsoever to do with female supremacy in order to discredit the movement and keep women "in their place." This is exactly like those who complain that we should be saying All Lives Matter. It's intentionally perverting the true goals of the movement in order to disempower it in the exact same way that BLM has been unfairly attacked and demonized.

There is absolutely no mainstream feminist movement that seeks female supremacy. All definitions from centrist sources talk about equality, "Feminism is a range of socio-political movements and ideologies that aim to define and establish the political, economic, personal, and social equality of the sexes." That's from Wikipedia.

It is a feminist movement because it is seeking to redress the systemic disempowerment of women - something that is still very much alive and well.